In a heatproof bowl over simmering water (or in short bursts in the microwave), melt the milk chocolate and butter together. Stir until smooth.

Remove from the heat and stir in the dark brown sugar.

Add the egg, egg yolk, and vanilla extract, and beat until the mixture is thick and glossy.

Dissolve the instant coffee in the milk, then stir this into the chocolate mixture.

In a separate bowl, sift together the spelt flour, cocoa powder, and salt.

Gently fold the dry ingredients into the wet mixture until just combined. The batter should be thick and smooth.

Pour the batter into the prepared tin and smooth the top.

Bake for 30–32 minutes, or until a skewer comes out with a few moist crumbs but no raw batter.

Let the cake cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.

Stir in the cocoa powder until smooth and glossy.

Remove from the heat and beat in the icing sugar.

Add 2 tablespoons of milk and stir until smooth. Add more milk, a teaspoon at a time, until the frosting is spreadable.