Preheat the oven to 350°F, grease a 9-inch round cake pan, and line the bottom with a parchment circle.

Cream the butter and sugar together until light and fluffy, about 2 minutes.

Beat in the egg and vanilla until fully combined.

Whisk the flour, baking soda and salt together in a separate bowl, then add to the butter mixture and stir until the dough just comes together.

Divide the dough in half; tint one half red and the other pink with gel coloring, kneading each gently until the color is even.

Pinch pieces of each color and press them together in the pan in a rough marbled pattern, pressing the dough out to the edges in an even layer about 1/2 inch thick.

Bake for 20 to 22 minutes, until the edges are golden and set but the center still looks slightly soft; it firms as it cools.

Cool in the pan for 20 minutes, then run a knife around the edge and lift the cookie cake onto a rack to cool completely, about 30 minutes more.

Spread or pipe the frosting over the top in a swirl, then slice into wedges to serve.