Preheat oven to 350 degrees F (175 degrees C). Grease 24 muffin cups or line with paper muffin liners.
Beat cake mix, water, eggs, and oil in a bowl with an electric mixer on medium speed until batter is blended. Divide batter into prepared muffin cups and sprinkle with strawberry pieces.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 19 to 22 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.
Mix cocoa powder and melted butter in a bowl. Gradually beat confectioners' sugar, alternating with milk, into cocoa powder mixture with an electric mixer on medium speed. Add vanilla extract and continue beating until frosting is smooth. Spread frosting over cooled cupcakes.