Preheat the oven to 180° / 160° fan/ Gas Mark 4.
Grease and Flour the Tins and line the base of each with Baking Paper.
In a large mixing bowl sieve in the Self-Raising Flour, Dr.Oetrker Baking Powder and Dr.Oetker Cocoa Powder. Then add in the Butte, Caster Sugar, Eggs, Milk and whisk together until the mixture is smooth and fluffy. Next fold in the Dr.Oetker Chocolate Chunks.
Divide the mixture evenly between the two tins. Smooth the top of the cake, hollowing it slightly in the centre with the back of a spoon.
Bake for 30-40 minutes until fully cooked. Test this by placing a skewer into the middle of the cake. If it comes out clean the cake is cooked. If there is still some cake batter stuck to the skewer then return to the oven for another few minutes and test again. When the cake is ready, remove from the oven and allow to cool in the tin for a few minutes, then turn out onto a wire rack and leave until completely cold.
Colour the Dr.Oetker Vanilla Buttercream using the Dr.Oetker Green Extra Strong Gel Food Colouring and mix well.
Trim the top of the cake to make it level, then cut each sponge in half horizontally. Place one sponge onto a cake board. Use a palette knife and spread 2 heaped tablespoons of the Green Buttercream Icing on top, then stack another layer of cake on top and repeat thing step twice more. Then using the Dr.Oekter Chocolate Buttercream to cover the top and sides of the cake. Place in the fridge for 30 minutes.
Next make the Shamrock decorations. Break up Dr.Oetker White Chocolate bar and place into a dry heatproof bowl over a saucepan of barely simmering water. Stir until completely melted. Remove from the heat and add in a few drops of Dr.Oetker Green Extra Strong Gel Food Colour. Stir again until smooth and evenly coloured.
To make the Shamrock decorations. Place one Dr.Oetker Heart Marshmallow onto a fork, then dip into the coloured chocolate. Tap the fork on the side of the bowl to allow any access chocolate to drip off. Then use another fork to help place the Heart Marshmallow onto the tray lined with baking paper. Now dip another Heart Marshmallow into the Chocolate and place it on the left side of the first Heart Marshmallow are touching. Then dip another Heart Marshmallow and place it onto the right side of the first Marshmallow; again ensure the pointed end is facing towards the middle and both Marshmallows are touching.
Then place one halved breadstick onto a fork and dip into the green coloured White Chocolate. Tap the fork on the side of the bowl to allow the access to drip off. Then place it in between the two marshmallows at the bottom. Add 1/4 teaspoon of chocolate at the top of the breadstick and the two pointed ends of the marshmallows. On the breadstick sprinkle with Dr.Oetker sprinkles. Then make 9 more marshmallows shamrock decorations and leave in the fridge or freezer until the chocolate is completely hard.
Taking the remainder of the melted White Chocolate coloured Green place it in a piping bag in order to write "Happy St.Patricks day" on top of the cake.
Using the Shamrock marshmallows you can decorate the cake as above or arrange them along the side of the cake. But once finished decorating leave the cake in the fridge for 20 minutes until the cake is set.