Step 1

Preheat the oven to 350°F and grease a 9x2-inch round cake pan.

Step 2

Using an electric mixer with a paddle attachment, beat the butter and sugar until light and fluffy (about 3–5 minutes). Mix in the egg, egg yolk, milk, and extracts until well combined, scraping the bowl as needed.

Step 3

In a separate bowl, whisk together the flour, baking powder, cornstarch, and salt. With the mixer on low, slowly add the dry ingredients to the wet mixture until just combined. Fold in the sprinkles and white chocolate chips.

Step 4

Press the dough evenly into the pan.

Step 5

Bake in the center of the oven for 23–25 minutes, until the edges are lightly golden. Place the pan on a wire rack and let the cookie cool completely in the pan.

Step 6

Remove the cooled cookie from the pan and decorate with frosting.

Step 7

Beat the butter with a whisk attachment on medium-high speed for about 3 minutes until light and fluffy. Mix in the vanilla.

Step 8

With the mixer on low, gradually add powdered sugar, one cup at a time, scraping the bowl as needed. Add the whipping cream and salt, then beat on medium-high for 3–5 minutes until light and fluffy. Adjust consistency by adding more cream if too thick or more sugar if too thin.

Step 9

Decorate as desired! I used a piping bag with a Wilton #5 tip to write "Happy Birthday" and a Wilton 1M tip for the edges. Finish with extra sprinkles—because there’s no such thing as too many!