Cream the butter and sugar. In a large bowl, beat the butter and granulated sugar on medium speed until pale and fluffy, about 2 to 3 minutes. Scrape down the bowl.

Add egg, yolk, and extracts. Beat in the egg and extra yolk until smooth, then mix in the vanilla and almond.

Combine dry ingredients. In a separate bowl, whisk flour, cornstarch, baking powder, and salt.

Bring it together. Add the dry mixture to the butter bowl in two additions, mixing on low just until nearly combined. Add sour cream and mix briefly until a soft dough forms. Avoid over-mixing.

Scoop and flatten. Line baking sheets with parchment. Scoop dough in 3 tablespoon portions and roll into balls. Press each to about 1/2 inch thick for that classic thick cookie look.

Optional chill. For sharper edges, chill the shaped cookies 20 to 30 minutes.

Bake. Bake at 350 F for 10 to 12 minutes, rotating the pan once. The tops should look set and pale with no browning.

Cool. Let cookies rest on the pan for 5 minutes, then move to a rack to cool completely before frosting.

Make frosting. Beat butter until creamy. Add powdered sugar, 2 tablespoons cream, vanilla, almond, and a pinch of salt. Beat until smooth and fluffy, adding a little more cream if needed. Tint with a small amount of gel color.

Frost generously. Spread a thick layer on each cooled cookie with an offset spatula. Swirl the top and add sprinkles.