Preheat the oven to 170C Fan/Gas 4 and grease and line a 23cm/9inch round cake tin.
In a large bowl, sift together the flour, baking powder, bicarbonate of soda, salt ,ground hazelnuts, cocoa powder and sugar.
In a separate jug, lightly whisk together the milk, eggs and vanilla. Pour this into the dry ingredients, followed by the melted butter and mix together until fully combined.
Stir in the hot coffee before pouring the batter into the prepared tin and bake for 35-40 minutes or until a skewer inserted in the centre comes out clean.
To make the buttercream, beat the butter in a stand mixer or with an electric whisk for 3-5 minutes until pale and creamy. Add in the sugar, espresso powder and salt and continue to beat for another 3 minutes until the buttercream is thick and fluffy. Pour in the heavy cream and beat briefly until combined.
When the cake is completely cooled, level the top with a serrated knife if the cake has domed. Generously spoon on the buttercream and decorate with blackberries, chopped hazelnuts and fresh thyme.