Preheat oven to 350°F. Line a 9 x 9 metal baking pan with parchment paper.
Pour the cooled melted butter and brown sugar into a large bowl and stir until well combined.
Stir in the eggs and vanilla extract until completely combined. The mixture will be thick.
In a separate smaller bowl, sift together the flour, cornstarch, baking powder, and salt.
Add the dry ingredients to the wet ingredients, a little at a time, stirring until fully combined.
Towards the end of mixing, gently fold in the chopped Mini Cadbury Eggs.
Pour batter into the brownie pan (batter will be very thick.) Spread the batter in an even layer.
Top with some extra chopped Mini Cadbury Eggs for decoration.
Bake for 20-22 minutes, until a knife in the center comes out clean. You want the edges to be set and the middle of the pan to be light brown and shiny, but not jiggly.
Let cool for at least 10 minutes before slicing. Enjoy!