Step 1
Preheat oven to 350 degrees F (175 degrees C). Lightly spray 12 muffin cups.
Step 2
Heat butter and chocolate together in a small saucepan over low heat until melted about 5 minutes. Remove from heat; set aside to cool slightly about 5 minutes.
Step 3
Combine 1 cup plus 2 tablespoons flour baking soda and salt together in a small bowl.
Step 4
Mix milk vinegar and peppermint extract together in another small bowl.
Step 5
Beat sugar and eggs together in large bowl with an electric mixer on medium speed until pale about 3 minutes. Add melted chocolate and beat well to combine. Pour in 1/2 cup boiling water and beat until batter is well-blended. Add flour mixture; beat on low speed until incorporated. Add milk mixture and blend well. Fill muffin cups 3/4-full with batter.
Step 6
Bake in the preheated oven until a toothpick inserted into a cupcake comes out clean 25 to 30 minutes. Transfer cupcakes to wire rack and cool to room temperature about 45 minutes.
Step 7
Combine 2 tablespoons cold water and gelatin in a small saucepan over low heat. Stir constantly until gelatin dissolves about 5 minutes. Remove from heat and let cool slightly but not until set about 5 minutes.
Step 8
Combine 1 cup plus 3 tablespoons heavy cream 1/4 cup plus 2 teaspoons confectioners' sugar vanilla extract and green food coloring in a large bowl. Whip with an electric mixer on low speed until slightly thickened about 5 minutes. Beat in dissolved gelatin slowly. Increase speed to high and beat until frosting is stiff about 3 minutes.
Step 9
Spread frosting over cooled cupcakes. Remove and discard cream filling from chocolate sandwich cookies; crush cookies. Scatter cookie crumbs over cupcakes.