PREHEAT oven to 350° F. Line 12-cup muffin tin with paper or foil liners. Place one regular size sandwich cookie in each cup.
MIX flour, sugar, 1/3 cup cocoa powder, baking soda and salt together in large mixer bowl.
WHISK together water, butter and oil in small bowl; stir into flour mixture until incorporated. Whisk in buttermilk, egg and vanilla extract until thoroughly combined. Spoon batter evenly into cookie-filled cups, filling 2/3 full.
BAKE for 15 to 20 minutes or until wooden pick inserted in cupcake comes out clean. Cool in pan on wire rack for 15 minutes. Transfer to wire rack to cool completely.
COMBINE frosting with remaining 2 tablespoons cocoa powder; stir until smooth. Pipe or spread frosting onto cupcakes. Garnish each cupcake with snack-size sandwich cookie.