Line a baking tray with baking paper.
Preheat the oven to 170° C.
Beat the soft butter with the sugar until light and fluffy.
Stir in the eggs one by one.
Mix the flour, sifted cocoa powder, baking soda and salt.
Stir the flour mixture and buttermilk into the butter mixture.
Pour the mixture onto the prepared tray and smooth it out.
Bake in the preheated oven for 30 minutes.
For the glaze, chop the chocolate and melt it with the coconut oil over bain marie.
Spread evenly on the cake.
Decorate with sprinkles.
Leave to cool completely.