Preheat the oven to 160°C fan/180°C/gas 4. Whisk the butter and sugar together until smooth, then beat in the eggs one at a time.

Gently fold in the self-raising flour, cocoa powder and baking powder, taking care not to overmix, then stir in the lime zest and juice and the milk. Spoon the batter into a lined 20cm x 26cm roasting tin.

Transfer to the oven and bake for 25–30 minutes, until firm to the touch and a skewer inserted comes out clean.

Leave the cake in the tin for 5 minutes, then gently lift it out on to a wire rack and leave to cool completely.

For the ganache, heat the double cream and lime zest in a saucepan until just under boiling, then pour it over the chopped dark chocolate in a heatproof bowl. Let it sit for 2 minutes, then whisk vigorously with the sea salt flakes until smooth and glossy.

Spread the ganache over the chocolate cake, swirling it neatly into waves if you wish, and leave it to set before slicing and serving.

Any leftovers will keep well in an airtight tin in a cool room for 2 days.

FOR DIABETICS: Replace the soft dark brown sugar with 170ml date syrup, and use diabetic-friendly dark chocolate in the topping.