diff --git a/Gemfile b/Gemfile
index 9e3c9cda..61e9e125 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,6 +6,8 @@ gem 'yard', '~> 0.8', require: false
 
 # Test tools
 gem 'pry', '~> 0.10', group: :development, require: false
+gem 'pry-byebug'
+gem 'pry-stack_explorer'
 gem 'aruba', '~> 0.6', require: false
 gem 'rspec', '~> 3.0', require: false
 gem 'fivemat', '~> 1.3', require: false
diff --git a/middleman-core/lib/middleman-core/application.rb b/middleman-core/lib/middleman-core/application.rb
index 64b36bed..f3775a8e 100644
--- a/middleman-core/lib/middleman-core/application.rb
+++ b/middleman-core/lib/middleman-core/application.rb
@@ -271,10 +271,10 @@ module Middleman
       @extensions.activate_all
 
       run_hook :after_configuration
-      config_context.execute_after_configuration_callbacks
+      config_context.execute_callbacks(:after_configuration)
 
       run_hook :ready
-      @config_context.execute_ready_callbacks
+      @config_context.execute_callbacks(:ready)
     end
 
     def evaluate_configuration
@@ -299,10 +299,10 @@ module Middleman
       end
 
       # Run any `configure` blocks for the current environment.
-      config_context.execute_configure_callbacks(config[:environment])
+      config_context.execute_callbacks([:configure, config[:environment]])
 
       # Run any `configure` blocks for the current mode.
-      config_context.execute_configure_callbacks(config[:mode])
+      config_context.execute_callbacks([:configure, config[:mode]])
     end
 
     # Whether we're in server mode
diff --git a/middleman-core/lib/middleman-core/builder.rb b/middleman-core/lib/middleman-core/builder.rb
index f5563293..b4f789df 100644
--- a/middleman-core/lib/middleman-core/builder.rb
+++ b/middleman-core/lib/middleman-core/builder.rb
@@ -62,7 +62,7 @@ module Middleman
 
       # Run hooks
       @app.run_hook :after_build, self
-      @app.config_context.execute_after_build_callbacks(self)
+      @app.config_context.execute_callbacks(:after_build, [self])
 
       !@has_error
     end
diff --git a/middleman-core/lib/middleman-core/callback_manager.rb b/middleman-core/lib/middleman-core/callback_manager.rb
new file mode 100644
index 00000000..3bedc8b9
--- /dev/null
+++ b/middleman-core/lib/middleman-core/callback_manager.rb
@@ -0,0 +1,49 @@
+require 'hamster'
+require 'middleman-core/contracts'
+
+# Immutable Callback Management, complete with Contracts validation.
+module Middleman
+  class CallbackManager
+    include Contracts
+
+    Contract Any
+    def initialize
+      @callbacks = ::Hamster.hash
+    end
+
+    Contract RespondTo[:define_singleton_method], ArrayOf[Symbol], Maybe[Proc] => Any
+    def install_methods!(install_target, names, &block)
+      manager = self
+
+      names.each do |name|
+        method_name = block_given? ? block.call(name) : name
+
+        install_target.define_singleton_method(method_name) do |*keys, &b|
+          key_set = keys.unshift(name)
+          manager.add(key_set.length > 1 ? key_set : key_set.first, &b)
+        end
+      end
+
+      install_target.define_singleton_method(:execute_callbacks) do |keys, *args|
+        manager.execute(keys, args, self)
+      end
+    end
+
+    Contract Or[Symbol, ArrayOf[Symbol]], Proc => Any
+    def add(keys, &block)
+      immutable_keys = keys.is_a?(Symbol) ? keys : ::Hamster::Vector.new(keys)
+
+      @callbacks = @callbacks.put(immutable_keys) do |v|
+        v.nil? ? ::Hamster.set(block) : v.add(block)
+      end
+    end
+
+    Contract Or[Symbol, ArrayOf[Symbol]], Maybe[ArrayOf[Any]], Maybe[RespondTo[:instance_exec]] => Any
+    def execute(keys, args=[], scope=self)
+      immutable_keys = keys.is_a?(Symbol) ? keys : ::Hamster::Vector.new(keys)
+
+      callbacks = @callbacks.get(immutable_keys)
+      callbacks && callbacks.each { |b| scope.instance_exec(*args, &b) }
+    end
+  end
+end
diff --git a/middleman-core/lib/middleman-core/config_context.rb b/middleman-core/lib/middleman-core/config_context.rb
index db351c0f..7c644f70 100644
--- a/middleman-core/lib/middleman-core/config_context.rb
+++ b/middleman-core/lib/middleman-core/config_context.rb
@@ -1,4 +1,5 @@
 require 'rack/mime'
+require 'middleman-core/callback_manager'
 
 module Middleman
   class ConfigContext
@@ -14,10 +15,8 @@ module Middleman
       @app = app
       @template_context_class = template_context_class
 
-      @ready_callbacks = []
-      @after_build_callbacks = []
-      @after_configuration_callbacks = []
-      @configure_callbacks = {}
+      @callbacks = ::Middleman::CallbackManager.new
+      @callbacks.install_methods!(self, [:ready, :after_build, :after_configuration, :configure])
     end
 
     def helpers(*helper_modules, &block)
@@ -43,48 +42,6 @@ module Middleman
       instance_eval File.read(other_config), other_config, 1
     end
 
-    def ready(&block)
-      @ready_callbacks << block
-    end
-
-    def execute_ready_callbacks
-      @ready_callbacks.each do |b|
-        instance_exec(&b)
-      end
-    end
-
-    def after_build(&block)
-      @after_build_callbacks << block
-    end
-
-    def execute_after_build_callbacks(*args)
-      @after_build_callbacks.each do |b|
-        instance_exec(*args, &b)
-      end
-    end
-
-    def after_configuration(&block)
-      @after_configuration_callbacks << block
-    end
-
-    def execute_after_configuration_callbacks
-      @after_configuration_callbacks.each do |b|
-        instance_exec(&b)
-      end
-    end
-
-    def configure(key, &block)
-      @configure_callbacks[key] ||= []
-      @configure_callbacks[key] << block
-    end
-
-    def execute_configure_callbacks(key)
-      @configure_callbacks[key] ||= []
-      @configure_callbacks[key].each do |b|
-        instance_exec(&b)
-      end
-    end
-
     def set(key, default=nil, &block)
       config.define_setting(key, default) unless config.defines_setting?(key)
       @app.config[key] = block_given? ? block : default