Butter a 20 x 20cm brownie tin and line with baking parchment. Heat the oven to 180C/160C fan/gas 4. Combine the condensed milk, 2 tbsp espresso powder and a large pinch of sea salt in a small bowl. Set aside. Melt the butter and chocolate in a heatproof bowl over a pan of simmering water. Or, do this in the microwave in 20-second bursts. Once melted, leave to cool slightly.

Beat the eggs and sugar using an electric whisk until pale and doubled in volume. Slowly pour in the chocolate mix, folding it in, then fold in the flour, cocoa powder, the 1 tsp espresso and a pinch of salt. Pour into the tin, smooth out, and use the back of a spoon to make nine equally spaced dips in the surface. Divide the condensed milk mixture between them, then use a skewer to swirl it, creating a marbled effect. Bake on the middle shelf of the oven for 20-25 mins until cracked and shiny with a slight wobble in the middle. Leave to cool completely in the tin, then cut into squares. Will keep in an airtight container for five days, or in the freezer for three months.