Preheat oven to 200 C / Gas 6. Line a muffin tin with 12 cases.
Sift together the flour, cocoa powder, baking powder and salt into a large bowl. stir in the sugar.
Combine eggs, cream and oil. Make a well in the centre of the dry ingredients and pour in the wet ingredients. Stir gently until just combined, do not over-mix.
Break the 85g of chocolate evenly into 12 pieces. Spoon the muffin mixture into the muffin cases, filling each only halfway. Place a piece of chocolate in the middle of each, then spoon in the remaining muffin mixture on top of the chocolate.
Bake in the preheated oven for 20 minutes until well risen and firm when touched.
Meanwhile, make the sauce. Melt the chocolate and butter together in a heatproof bowl set over a pan of gently simmering water. Stir until smooth, then stir in the cream. Remove from the heat and stir until smooth.
Once the muffins are out of the oven, leave for five minutes to cool slightly, then remove from the tin and place on serving plates. Now just pour the sauce over the muffin and enjoy!