Put the butter and chopped dark chocolate in a large, heatproof bowl. Melt together to give a smooth, cohesive mixture. You do this using 30 second bursts in the microwave or over a bain marie (placing the bowl over a pan with a little simmering water and allowing the steam to heat the bowl as you stir gently).
Add the sugar and stir together - it might seem grainy.
Add the vanilla and beaten eggs. Whisk together - again you're looking to mix just until you have a smooth cohesive mixture, without introducing too much air.
Sieve the flour into the bowl. Stir together with a silicone spatula or metal spoon, making sure there are no pockets of flour at the bottom of the bowl.
Grease and line a 20cm x 28cm (8 inch x 11 inch) non-stick shallow rectangular cake tin.
Pour two thirds and the brownie batter into the tin and level off. Drop 18 mini creme eggs on top.
Pour the remaining brownie batter on top and level off.
Halve the remaining mini creme eggs and full size creme and arrange all over the top of the mixture, cut side up.
Bake for 30-35 minutes. You can test if it's baked by pushing a skewer into the centre. If under-baked, you'll see wet batter on the skewer. If just right, you'll see sticky crumbs. If over-baked, the skewer may come out completely clean. Place on a wire rack to cool for 15 minutes, then turn out to cool completely.
Once completely cooled, transfer the brownie to a board and cut into 12 equally sized squares in a 3x4 pattern. For really clean brownie slices, I like to chill my brownies in the fridge overnight before slicing. Enjoy the chocolatey Cadbury Creme Egg brownie goodness!