Preheat the oven to 160C/fan 140C/gas 4. Base line a 20cm round and a 18cm square cake tin.
Put the butter and chocolate into a saucepan and stir over a very gentle heat until melted and smooth. Remove from the heat and stir in the vanilla extract.
Sieve the flour, baking powder and bicarbonate of soda into a large bowl. Add the sugar, breaking up any lumps if necessary. Beat together the eggs and soured cream and pour onto the flour mixture. Add the melted chocolate and mix all well together until smooth.
Divide the mixture equally between the two tins and bake in the oven for about 55 mins - 1 hour. A good way to check that the mixture is equally divided is to put a cocktail stick into each mixture and the levels should be the same.
To make the icing, put the butter into a bowl and beat until creamy. Add the icing sugar and cocoa a little at a time beating well between each addition. Add the milk to mix to a soft spreading consistency.
To assemble, place the round cake on a large wooden board or tray (the cake will need to served on this). Place the round cake tin on the square cake about 12cm from one edge and cut around the edge of the tin. Remove the smaller piece and place at one end of the round cake to represent the ears.
Using the remains of the square cake cut almost in half to give an oblong (for the fence) and a semi circle (for the wings). Place the oblong at the bottom of the owl cake. Slice the semi circle in half horizontally through the middle. Arrange one piece each side of the round cake to make the wings.
Spread the butter icing all over the cake. Cut the flake bars into small pieces and arrange on the wings to represent feathers. Keep the flake crumbs. Arrange milk chocolate buttons over the body. Place chocolate fingers on the bottom cake to make the fence. Using a fork, mark the icing to make the owl’s ears. Use the flake crumbs to make a v-shape between the ears like a 'widow’s peak'. Arrange the white chocolate buttons in two circles to make the eyes and place a milk chocolate button in the centre of each. Use a third of the marzipan to shape a beak and use the remaining marzipan to make two feet.
Place the candles on the ‘fence’ part of the cake. You can also pipe a child’s name or message on the fence using writing icing.