01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing overhang for easy removal.
02 - Whisk together flour, baking powder, and salt in a bowl. Set aside.
03 - Combine melted butter and brown sugar in a large mixing bowl. Stir until smooth and fully incorporated.
04 - Beat in egg and vanilla extract until mixture is uniform and smooth.
05 - Gently fold dry ingredients into wet mixture with a spatula until just combined. Avoid overmixing to prevent dense texture.
06 - Fold in white chocolate chips, milk chocolate chips if using, and chopped Cadbury Mini Eggs until evenly distributed.
07 - Spread batter evenly into prepared pan. Press reserved whole Mini Eggs onto surface, embedding slightly into batter.
08 - Bake for 22-25 minutes until top is golden and toothpick inserted in center emerges with moist crumbs.
09 - Cool completely in pan. Lift out using parchment paper overhang and cut into 12 squares.