Preheat the oven to 180 ̊C/ 350 ̊F fan/ Gas Mark 4. Grease and line the cake tins.
For the cake, cream the sugar and butter until pale and fluffy. Beat in eggs, one at a time, until combined. Stir in the vanilla extract and enough drops of green food colouring to dye the batter green.
Combine the flour and the baking powder in a small bowl. Beat this, alternating with the milk, into the butter mixture.
Divide the batter between the cake tins and bake for 30-40 minutes until a skewer inserted the centre comes out clean. Leave to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
For the buttercream, beat the butter and icing sugar together on a low speed until combined, then beat on high speed for three minutes until fluffy. Beat in the vanilla extract and the cream until blended.
Beat in a few drops of green food colouring until the buttercream turns a bright green shade.
Place one of the cakes, top-side down, on your cake board and spread the top with some buttercream. Add the other cake, also top-side down, and then spread the entire cake with the rest of the buttercream.
Divide one pack of the ready to roll Icing in half. Roll one half into a 20cm circle, then use a rolling pin to transfer it over the cake; it should just cover the top of the cake, so use a small, sharp knife to trim away any of the excess.
Add drops of black food colouring to the other half of icing, then knead until it is a deep, even colour. Roll out the icing, then use the knife to cut out small six-sided shapes. Brush a small bit of water onto the back of the shapes, then secure them to the top of the cake.
Knead the other pack of ready to roll Icing until it is soft, then add a few drops of green food colouring, Knead this until it is a bright green shade.
Roll this out into an 80cm long rectangle and trim the sides so it is even. Wrap this around the rolling pin and transfer it to wrap around the sides of the cake.
Let it come a little over the top rim of the cake, then use a scissors to make small snips in the icing to resemble blades of grass.