Heat the oven to 180°C (160° fan) 350°F gas 4.
Line 3 large baking trays with non-stick baking paper.
Mix together the flour, cocoa powder, bicarbonate of soda and salt.
Beat the butter in a mixing bowl until soft and creamy.
Beat in the sugar until light and fluffy.
Beat in the egg and vanilla until smooth.
Add the flour mixture alternatelywith the buttermilk, beating until just combined.
Stir in the food colouring.
Drop 2 tablespoons of the mixture at a time onto the baking trays, keeping them about 4 cm apart and spread out a little to 7,5 cm diameter rounds.
Bake for 10-15 minutes, until firm to the touch.
Cool on the baking trays for a few minutes, then place on a wire rack to cool completely.
Put all the ingredients in a large heatproof bowl and whisk until well mixed.
Place the bowl over a pan of boiling water, then whisk constantly for 5-7 minutes or until the mixture is thick and creamy and stands in peaks.
Remove from the heat, whisk for 1 minute, then allow to cool completely before using.
Spread the flat side of a cold cake with a generous amount of filling.
Place another half, flat side down, on top of the filling then press both halves together lightly.