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

2. In a large bowl, whisk melted butter with granulated sugar and brown sugar until well combined and glossy, about 1 minute.

3. Add eggs one at a time, whisking vigorously after each addition until mixture is thick and lightened in color, about 2-3 minutes total.

4. Stir in vanilla extract until combined.

5. In a separate bowl, whisk together cocoa powder, flour, salt, and baking powder until no streaks remain.

6. Pour dry ingredients into wet mixture all at once. Fold gently with a rubber spatula just until no dry flour remains visible.

7. Scrape batter into prepared pan and spread evenly to corners. Tap pan on counter 3-4 times to release air bubbles.

8. Bake for 28-32 minutes until a toothpick inserted 2 inches from edge shows moist crumbs but no wet batter.

9. Remove from oven and cool in pan on wire rack for 15 minutes.

10. Use parchment handles to lift brownies onto cooling rack. Cool completely, at least 2 hours.

11. For frosted variations: Beat cream cheese with powdered sugar and vanilla until fluffy. Divide and tint with food coloring as desired.

12. Spread frosting over cooled brownies using offset spatula.

13. Add decorations while frosting is soft. Create your chosen variation using candy eyes, sprinkles, or themed candies.

14. Chill decorated brownies for 30 minutes to set decorations before cutting.

15. Cut into squares using a sharp knife wiped clean between cuts.