Preheat oven to 350°F. Line two baking sheets with parchment paper.
Whisk flour, cornstarch, baking powder, baking soda, and salt in a medium bowl.
Cream butter and sugar with a mixer until very light and fluffy, 2 to 3 minutes.
Beat in eggs one at a time. Mix in vanilla and almond extract.
Blend in sour cream until smooth and creamy.
Add dry ingredients in two additions. Mix on low just until combined.
Rest dough 10 minutes to hydrate. Chill briefly if sticky.
Scoop 1.5-tablespoon mounds onto sheets, spacing about 2 inches apart.
Flatten gently to roughly 1/2 inch thick for even baking.
Bake 10 to 12 minutes until set but still pale. Cool completely on racks.
Beat frosting butter until creamy. Add powdered sugar, milk, vanilla, and salt; whip until fluffy.
Tint frosting if desired. Spread generously on cooled cookies and finish with sprinkles.
Notes
For a bakery-style look, tint the frosting pastel and add festive sprinkles. For a strawberry twist, beat in 1–2 tbsp finely ground freeze-dried strawberries to the frosting. Store frosted cookies airtight at room temperature for 2 days, or refrigerate up to 5 days. Freeze unfrosted cookies up to 2 months; thaw and frost before serving.This recipe is an original creation inspired by classic Lofthouse Cookies with Frosting flavors. All ingredient ratios and instructions are independently developed.