1. Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving overhang on two sides. Lightly grease parchment.

2. In a medium bowl, whisk together flour, baking powder, and salt. Whisk vigorously for 30 seconds. Set aside.

3. In a large bowl using electric mixer, beat softened butter on medium speed for 1 minute until smooth. Add sugar and beat on medium-high for 2-3 minutes until light, fluffy, and pale.

4. Add eggs one at a time, beating well after each addition (about 30 seconds per egg). Add vanilla and almond extract (if using). Beat until fully incorporated.

5. With mixer on low speed, gradually add flour mixture in three additions. Mix just until no flour streaks remain after each addition. Don’t overmix.

6. Transfer dough to prepared pan. Using greased hands or offset spatula, press and spread dough evenly into every corner. The layer should be uniform thickness (about ½ inch). Smooth with bottom of measuring cup.

7. Bake for 18-22 minutes until edges are barely golden and center looks set but is still soft. A toothpick should come out with moist crumbs. Don’t overbake.

8. Cool in pan for 15 minutes, then use parchment overhang to lift onto cooling rack. Cool completely before frosting (at least 1 hour, preferably 2 hours or overnight).

9. Make frosting: Beat softened butter for 1 minute. Add 3 cups powdered sugar, 3 tablespoons milk, vanilla, and salt. Beat on low until combined, then medium-high for 2-3 minutes until fluffy. Adjust consistency with more milk or powdered sugar.

10. Spread frosting evenly over cooled bars using offset spatula. Immediately add sprinkles or decorations.

11. Let frosting set for 30 minutes. Cut into 24 bars (6 rows by 4 rows) using a sharp knife warmed under hot water and wiped clean between cuts.