For the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with blue paper liners. Butter and flour an 8-inch round cake pan and line the bottom with a round of parchment.
Whisk together the flour, baking powder and salt in a bowl. Beat the butter in a separate bowl with a electric mixer on medium-high speed until smooth, 1 minute. Add the granulated sugar and beat until light and creamy, about 3 minutes. Beat in the eggs, 1 at a time, then beat in the vanilla.
Beat in the flour mixture in 3 batches on low speed, alternating with the milk in 2 additions, then beat on medium-high speed until just combined.
Spread half of the batter in the cake pan and divide the remaining half among the paper liners. Bake until the tops spring back and a tester inserted in the centers comes out clean, 20 to 25 minutes for the cupcakes and 30 to 35 minutes for the cake layer. Let cool 5 minutes in the pans, then remove to a baking rack to cool completely.
For the buttercream: Beat the butter in a bowl with an electric mixer on medium-high until light, about 1 minute. Add the confectioners' sugar, vanilla and salt and beat on low to combine, then increase the speed to medium-high and beat until light and creamy, about 3 minutes. Beat in the milk, a tablespoon at a time, to make a spreadable frosting. Add the food coloring, a drop at a time, to make a medium blue color.
Frost the cupcakes with about half of the buttercream. Halve the cake layer crosswise. Cut a small "V" from the middle of the flat side of one piece to make a fish tail. Frost both layers with the remaining buttercream.
For the decoration: Shingle the M&Ms on one half of each cupcake to make the fish scales (you will have enough for 2 cupcakes per color, with some of each leftover for the tail). Add a candy eyeball to each. Insert 2 red cinnamon candies on the opposite side from the scales to make fish lips. Halve 6 of the white gumdrops crosswise, then halve those pieces lengthwise to make half-moons. Stick 2 pieces of white gumdrop at the top and bottom of each cupcake for the fins.
Stick a white gumdrop near the top of the cake fish head. Top with a black M&M, using a little frosting to make it adhere.
Halve the red gumdrop crosswise. Halve one piece lengthwise to make half-moons and stick to the rounded side of the cake fish head for lips. Set the head at the left side of your serving tray. Follow the head with the cupcakes to make the fish body. Fit the tail on the tray at the end. Shingle the remaining M&Ms on the tail in stripes to make fish scales.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
Copyright 2017 Television Food Network, G.P. All rights reserved.