Line a 20 x 30cm traybake tin with 2 sheets of cling film (in a criss-cross pattern). Put the chocolate and butter in a large bowl set over a saucepan of gently simmering water, and melt until smooth and glossy.
Remove from the heat and add the cocoa powder and golden syrup. Mix together until fully combined and leave to cool at room temperature for about 15 mins.
Put the biscuits in a freezer bag and use a rolling pin to bash them, leaving some pieces chunkier than others. Stir into the cooled chocolate with the marshmallows, cranberries and 150g of the mini eggs.
Pour the mix into the tin and press down with the back of a spoon until even. Scatter over the remaining mini eggs, pressing them in a little, and leave to set in the fridge for 1 hr.
Remove from the tin and cut into bars to serve. Will keep for up to 1 week in an airtight container.