Preheat the oven to 350°F/180°C, and line a baking sheet with parchment paper.
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.
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.
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.