Mix the sugar, flour, cocoa powder, bicarb and baking powder together, breaking up any lumps of sugar or cocoa powder. In a separate bowl, whisk together the eggs, buttermilk, coffee, oil, tahini and vanilla extract. Make a well in the dry goods and pour in the egg mixture. Use a whisk to fold the two mixtures together, whisking very briefly to make a smooth batter. Pour into the prepared tin, put on a baking tray and bake for 45-50 minutes, or until a skewer inserted into the cake comes out clean. Allow the cake to cool in the pan for 10 minutes before inverting onto a wire rack to cool completely.