For the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with red paper liners.
Whisk together the flour, baking powder and salt in a bowl. Beat the butter in a separate bowl with an electric mixer on medium-high speed until smooth, about 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.
Divide the batter among the paper liners. Bake until the tops spring back and a tester inserted in the centers comes out clean, 20 to 25 minutes. Let cool 5 minutes in the pan, then remove the cupcakes to a baking rack to cool completely.
For the buttercream: Beat the butter in a bowl with an electric mixer on medium-high speed until light, about 1 minute. Add the confectioners' sugar, vanilla and salt and beat on low to combine, then increase 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.
Reserve about 1/3 cup frosting and transfer the rest to a piping bag fitted with a large star tip. Pipe the frosting on the cupcakes.
For the decoration: Top the cupcakes with the crushed rock candy to resemble waves.
Split the macarons open. Stick 2 halves of the flat gummies near the top at 10 and 2 o'clock of each macaron for the crab claws. Press 3 licorice pieces under each claw for the crab legs. Spread a 1/2- to 1-teaspoon dollop of frosting on top to adhere the appendages and sandwich the macaroon back together. Make sure the claws and legs are stuck into the frosting. Glue 2 small round red gummy candies to the top of each macaron using a little more frosting. Stick an eye on top of each red gummy candy with a little more frosting. Press a macaron crab on top of each cupcake.
For the beach: Spread the turbinado sugar on a serving tray to resemble sand. Line the rock candy along one edge to resemble the surf. Stick the crabs in the sand, adding the gummy sea creatures to the surf and sand.
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.)
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