Tip the butter into a heatproof bowl with the dark chocolate. Fill a small pan about a quarter full with boiling water, then sit the bowl on top, not touching the water. Put over a low heat until the butter and chocolate have melted, stirring occasionally. Remove the bowl from the pan and leave the melted mixture to cool to room temperature.

Heat the oven to 180C/160C fan/gas 4. Line a shallow 20cm square brownie tin. Using an electric whisk, whisk the eggs and sugar until they are thick and creamy and doubled in volume. This can take 3-8 mins, depending on how powerful your whisk is. Pour the cooled chocolate mixture over the eggy mousse, then gently fold together with a spatula, moving the bowl round after each folding so you can get at it from all sides, until the two mixtures are one and the colour is a dark brown. Try not to knock the air out.

Sieve the flour and cocoa over the eggy chocolate mixture along with a good pinch of salt. Gently fold in this powder as well as the espresso mixture using the same figure-of- eight action as before. The mixture will look dry and dusty at first; keep going until it looks fudgy, but don’t overdo this mixing. Pour the mixture into the tin and gently ease the mixture into the corners of the tin and paddle the spatula from side to side across the top to level it. Push the chocolate balls into the brownie at regular intervals.

Bake for 30 mins or until the top has a shiny, papery crust and the sides are just beginning to come away from the tin. Remove from the oven. If you are serving these as a dessert, leave to cool a little.

To make the sauce, heat the coffee and sugar together until the sugar dissolves. Mix a little of the cream with the cornflour, then add to the pan with the remaining cream. Bring to a simmer and stir over a medium heat until thickened. Add the vodka and turn off the heat. Serve each square of brownie with the sauce poured over.