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Heat oven to 180C/160C fan/gas 4. Chop 4 of the chocolate biscuits into small pieces and set aside. With the rest of the biscuits separate the filling from the biscuits, don’t worry if the biscuits snap when you do this – they’re going to get blitzed up anyway. Reserve the filling and whizz the biscuits to a fine crumb in a food processor. Pour the melted butter over the biscuit crumbs and pulse briefly just to mix. Tip into a 20cm x 30cm tin and press the mixture down with the back of a spoon to cover the base of the tin in an even layer. Bake in the oven for 10 mins to firm up.
Put the dark chocolate in a bowl set over a pan of barely simmering water until melted, or melt in the microwave, checking after each 30 second blast. Leave to cool to room temperature.
Meanwhile put the butter and sugar in a bowl and beat until light and fluffy. Add 4 of the eggs, one at a time, beating well between each addition, then sift in the flour, cocoa, and a pinch of salt. Now stir in the melted chocolate and the chopped milk chocolate. Scrape the mixture into your prepared tin and smooth the surface.
Mix the reserved biscuit fillings with the remaining egg until smooth then stir in the cream cheese until it’s a lump-free as you can get it – try not to overmix. Put teaspoonfuls of the cream cheese mixture on top of the brownie layer, then scatter over the chopped biscuit pieces from earlier. Bake for 30 mins then add the chocolate mini eggs, cut side up, and pop it back in the oven for 5 more mins or until just set – it should still have a little wobble when it comes out of the oven, and a skewer inserted into the centre should come out with sticky crumbs. Cool completely in the tin before cutting into squares to serve.