Heat oven to 180C/160C fan/gas 4. Line 24 holes of a mini muffin tray with paper cases. Put the butter, dark chocolate and sugar in a pan and heat it very gently, stirring all the time until the butter and chocolate have melted. Remove from the heat and leave to cool for a few mins.
Meanwhile, sift the flour, cocoa and a good pinch of salt into a large bowl. Stir in the warm, melted chocolate mixture and the beaten eggs, then add half the chocolate chips and mix until just combined.
Divide the mixture between the cases and place a mini egg into the middle of each muffin, pushing down gently. Bake for 12-15 mins until cooked but still gooey in the centre – they will continue cooking a little as they cool down. Leave to cool for 10 mins in the tin before transferring to a wire rack to cool completely.
Melt the rest of the chocolate chips in short bursts in the microwave, or in a bowl set over a pan of simmering water, stirring frequently. Leave to cool until it is quite thick, then dot a small amount on each cake and stick on some more mini eggs.