Preheat the oven to gas 4, 180C, fan 160°C Grease and line 3 x 18cm round tins. In a blender, puree 100g of the strawberries, then remove the pips by passing through a fine sieve. Set aside.
Using an electric hand whisk, beat 180g of the butter until smooth. Add the caster sugar and 1tsp vanilla extract and beat until light and fluffy, then beat in half the buttermilk. Add the eggs one at a time, beating well, then fold through the flour and bicarbonate of soda. Stir in the remaining buttermilk.
Divide the batter between 3 bowls. Fold the melted chocolate into one bowl, and 1tsp vanilla extract into the second. Mix the strawberry puree and pink food colour into the third. Pour the batters into the 3 lined tins. Bake for 20-25 minutes, until a skewer comes out clean.
To make the buttercream, whisk together the icing sugar, remaining butter, vanilla extract and 6tbsp milk, until of a smooth spreading consistency. Add an extra 1tbsp milk if necessary.
Level the sponges using a serrated knife, then sandwich the cakes together by spreading 200g buttercream between each layer, with the chocolate sponge on the base, the vanilla sponge in the middle and the strawberry sponge at the top.
Using a knife, coat the cake with the remaining buttercream. Sweep around the cake with a palette knife held at a 45 degree angle to remove the excess icing and reveal the sponge beneath. Decorate with the grated chocolate and remaining strawberries, and add bunting.