Preheat oven to 350 degrees. Combine cocoa and 2 cups boiling water in a large heatproof bowl, stirring until blended and smooth; cool completely. Beat butter with a mixer until creamy; gradually add sugar, beating until blended. Add eggs, 1 at a time, beating until blended after each addition. Combine flour and next 3 ingredients; add to butter mixture alternately with cocoa mixture, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in chocolate extract.
Place paper baking cups in 2 (12-cup) muffin pans, and coat with cooking spray; spoon batter into cups, filling two-thirds full. Bake for 12 to 15 minutes. Cool in pans on wire racks 10 minutes; remove from pans, and cool completely.
To prepare Chocolate Frosting, beat first 4 ingredients at medium speed with an electric mixer until creamy. Gradually add powdered sugar, beating at low speed until blended. Beat at high speed 2 minutes or until creamy. Makes 3 cups.
Frost each cupcake with Chocolate Frosting using metal tip no. 2D. Top each with about 1 Tbsp. chocolate chunks.