• Preheat the oven to 350°F. Spray a 9-inch springform pan with cooking spray, line with a parchment paper round, then spray the parchment paper.
• In the bowl of a stand mixer, or using a hand mixer, cream together the butter, brown sugar and granulated sugar until light and fluffy, about 2-3 minutes.
• Add the egg, egg yolk and vanilla and mix until well-combined.
• Mix in the gluten-free flour, salt and baking powder until evenly combined. Stir in the chocolate chips.
• Spread the dough into the prepared baking pan. Top with a few more chocolate chips, if desired.
• Bake for 22-26 minutes, until the edges start to turn golden.
• Remove the cake from the oven and let cool in the pan for 15 minutes. Remove the cake from the pan and transfer to a wire rack to cool completely before frosting.

For the chocolate buttercream frosting:

• In the bowl of a stand mixer, or using a hand mixer, beat the butter on medium-high speed until creamy, about 2 minutes. Add the powdered sugar along with the cocoa powder, heavy cream or milk, vanilla and salt. Mix on low speed for 30 seconds, then increase to medium-high speed and whip for a full 2 minutes.
• If the frosting is too thick, add a little more heavy cream or milk. If the frosting is too thin, add more powdered sugar to get a pipeable consistency.
• Pipe a border of frosting around the edge of the cookie cake. Top with sprinkles.