Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper or grease lightly.
In a large bowl, whisk together melted butter and sugar until well combined.
Beat in eggs and vanilla.
Add cocoa powder, flour, salt, and baking powder. Stir until just combined.
Fold in chocolate chips and half of the chopped Cadbury Mini Eggs.
Pour batter into the prepared pan and smooth the top.
Sprinkle the remaining Mini Eggs on top.
Bake for 25–28 minutes or until the edges are set and the center is slightly gooey.
Cool completely before slicing for clean edges (or enjoy warm and gooey!).