Preheat oven to 350°F (175°C). Line a 9x9-inch square baking pan with parchment paper, leaving overhang on two sides. Lightly grease if desired.

In a microwave-safe bowl, combine butter and chopped semi-sweet chocolate. Microwave in 30-second intervals, stirring between, until smooth. Set aside to cool slightly.

In a large bowl, whisk together granulated sugar, brown sugar, eggs, and vanilla until pale and thickened, about 2 minutes.

Pour the melted chocolate mixture into the egg mixture and stir until combined. Sift flour, cocoa powder, and salt over the batter. Fold gently with a spatula just until no dry streaks remain. Do not overmix.

Pour batter into prepared pan and spread evenly. Bake for 22–28 minutes, or until edges are set and a toothpick inserted in the center comes out with a few moist crumbs. Cool completely in pan on a wire rack.

Once cooled, lift brownies from pan using parchment handles. For clean cuts, chill for 30 minutes before slicing into 16 squares.

Melt pastel candy melts according to package directions. Drizzle over brownies using a spoon or piping bag. Immediately press mini chocolate eggs into the drizzle and sprinkle with pastel sprinkles. Allow decorations to set before serving.