Step 1: Preheat your oven to 180°C (350°F) and grease and line an 8×8 inch square baking tin with parchment paper.
Step 2: In a large bowl, whisk the melted butter with the granulated and brown sugars until well combined and slightly cooled.
Step 3: Add the eggs and vanilla extract to the mixture. Whisk vigorously for 3-5 minutes until the batter is pale and slightly thickened; this ensures a crackly top.
Step 4: Sift in the flour, cocoa powder, and salt. Gently fold the dry ingredients into the wet mixture using a spatula until just combined.
Step 5: Pour the batter into the prepared tin and bake for 20 minutes. While baking, slice your frozen Creme Eggs in half lengthwise.
Step 6: Remove the brownies from the oven and quickly press the halved eggs into the top of the batter, fondant side up. Return to the oven for another 10 minutes.
Step 7: Remove from the oven and allow the brownies to cool completely in the tin before lifting them out and slicing. For the best results, chill in the fridge for 2 hours before cutting.