Line an 8” square tin with parchment. Preheat the oven to 180C/350F.
Place 200g of mini eggs into a sealed freezer bag and bash with a rolling pin into large chunks. Set aside.
Melt the butter in a small bowl, then add in the cocoa and whisk together until silky.
In a separate large bowl, beat the eggs and Tate & Lyle caster sugar until pale and creamy, about 5 minutes. Drizzle in the chocolate mixture, beating to evenly mix. Stir in the vanilla.
Sift in the flour, baking powder and salt. Fold in gently so you don’t lose the volume from the mixture. Tip in the mini eggs crushed earlier.
Tip the mixture into the tin, spreading out evenly into the corners. Bake for 25-35 minutes until the edges feel firm and the middle has the tiniest wobble.
Remove from the oven and immediately push the remaining mini eggs into the top of the brownie. Leave to cool in the tin before cutting into squares. Enjoy!