Heat the oven to 180C/160C fan/gas 4. Butter the base of two 20cm springform cake tins and line with baking parchment. Melt the butter and chocolate in a heatproof bowl over a pan of simmering water (make sure the base of the bowl doesn’t touch the water), or do this in the microwave in 10-second bursts. Set aside. Put the flours, cocoa powder, baking powder, bicarbonate of soda, sugar and 1/2 tsp salt in a bowl, and mix well. If there are any lumps in the sugar, squeeze these through your fingers to break them up.

Put the yogurt, coffee, and vanilla in a jug, and whisk in the eggs until smooth. Pour the wet ingredients, along with the melted chocolate mixture, into the dry ingredients. Fold everything together until well combined.

Divide the cake batter evenly between the two tins, and bake for 30 mins, or until risen and a skewer inserted into the centres comes out clean. Leave to cool in the tins for 10 mins, then turn out onto a wire rack, peel off the baking parchment and leave to cool completely. Once cooled, the cakes will keep, well wrapped, for up to three days.

To make the icing, melt the chocolate as you did in step 1. Cool slightly. Put the butter, icing sugar and cocoa in a large bowl and mash together, then beat with an electric whisk until smooth. Add the melted chocolate and milk, then mix until smooth.

Split the cakes in half through the equator using a bread knife. Secure one layer to a board or plate using a little icing. Spread over a layer of icing, sprinkle over a little sea salt, then sandwich with another cake layer. Repeat until about half the icing and all the remaining cake layers have been used, ensuring the final layer is flat-side up. Spread the remaining icing all over the cake using a palette knife to smooth it, or swipe upwards around the side for texture. Dust with cocoa powder and sprinkle with sea salt flakes. Will keep covered for up to four days.