• Preheat the oven to 350°F/180°C (160°C for a fan oven), and line a baking sheet with parchment paper or a Silpat silicone baking mat. Set aside.
• In a medium mixing bowl, combine flour, baking soda, baking powder, and salt. Set aside.
• In a separate mixing bowl, with a hand mixer, cream the butter and sugar until light and fluffy. Add egg, egg yolk, and vanilla extract, and beat again.
• Add the flour mixture, a little at a time, mixing after each addition. If the dough gets too thick for your mixer, finish mixing the dough by hand.
• Use a medium cookie scoop to drop the dough onto the lined baking sheet, leaving two inches between each cookie.
• Bake for 9-11 minutes, or until the edges are lightly golden brown.
• Allow the cookies to cool on the pan for 2 minutes, then transfer to a wire rack to cool completely.

For the Frosting:

• In a large bowl with a hand mixer, or in the bowl of your stand mixer, beat the butter at medium speed for 1 minute, or until creamy.
• Decrease the speed to low, and gradually add in the powdered sugar, vanilla extract, and salt. Continue beating for 3 minutes until light and fluffy.
• If needed, add a small amount of milk to the frosting to get it to the perfect piping consistency.
• Divide the frosting in half into two separate bowls. Use food coloring to tint one bowl light blue, and the other a light teal.
• To combine the frostings, lay out two pieces of plastic wrap. Add the frosting to the wrap, and then roll it up into a tube shape, twisting the ends to seal. Snip off one of the twisted ends from each tube.
• Add a star tip to a large piping bag, then add both colors of frosting, with the open end of the rolls pointing down toward the tip.
• Add a swirl of buttercream to each cookie, then top with a gummy shark.