1. Prep the pan and oven
Preheat the oven to 350°F (175°C). Line an 8×8-inch square pan with parchment paper or foil, leaving a bit of overhang for easy removal.

2. Make the cookie batter
In a mixing bowl, beat the softened butter with the granulated and brown sugars until smooth and slightly fluffy. Mix in the egg, then stir in the almond and vanilla extracts until evenly combined.

Whisk together the flour, cornstarch, baking powder, and salt, then fold the dry mix into the wet ingredients just until the flour streaks disappear. Press the dough evenly into the prepared pan.

3. Bake
Bake for about 15 minutes. The center may still look a touch underdone — that’s okay; the bars will finish setting as they cool. Allow the pan to cool on a rack until the surface is mostly set.

4. Make the frosting
In a clean bowl, beat the softened butter and powdered sugar until smooth. Add the heavy cream and vanilla and whip until light and airy. Adjust cream or sugar to reach your desired spreadable consistency.

5. Finish and serve
Spread the frosting over the cooled cookie slab and sprinkle generously with festive decorations. If you prefer firmer frosting, pop the pan into the refrigerator for 15–30 minutes before cutting. Use the parchment overhang to lift the whole block out, then slice into squares.