For the cake: Preheat your oven to 180C/160C Fan and grease and line a 12x9inch baking tin. In a small bowl, stir together the cocoa powder and freshly brewed black coffee to a thick paste without lumps. Then set aside to cool slightly.
In a large bowl, whisk together the cocoa paste, butter and sugar for 1-2minutes until thick and well combined. Then add the eggs one at a time, mixing well between each addition. Fold in the flour and baking powder until just combined. Then gently stir in the milk until combined. Pour into the lined tin, then dollop the tahini on top and swirl gently.
Bake for 30 minutes until a knife inserted into the centre of the cake comes out clean. Leave to cool in the tin for 10 minutes, then remove and leave on a wire rack to cool completely.
For the buttercream: Using an electric whisk beat the butter, icing sugar and salt together for 5-10 minutes until very pale (almost white), light and fluffy. Then add the tahini and whisk again to combine thoroughly. Using a palette knife, spread the buttercream evenly ontop of the cooled cake.
Cake can be kept in an air-tight container for 2-3 days.