So when I first started making cakes I always struggled to find recipes for a two tier birthday cake, I think it's because when using different tin sizes most people just use their normal recipes and adapt it. However if you're not to great at maths or don't have the time to practice here is how to do it.
Vanilla sponge recipe
6" requires 1 recipe (Fill two tin 1/2 full, use leftovers for cupcakes)
8" requires 1 recipe split between two tins
10" requires 2 recipes one per tin
400g plain flour
325g casting sugar
280g unsalted butter
5 large eggs
1/4 tsp salt
1/8 tsp cream of tartar
3tsp baking powder
2tsp vanilla extract
1. Preheat gas 4 (180C/350F). Make sure your eggs are at room temperature.
2. Line your cake tin with baking paper.
3. Separate your eggs one bowl for egg whites, one for egg yolks and put to the side.
4. Beat together your butter, 3/4 of your sugar leaving the other 1/4 to the side for the egg whites. Once smooth add your flour, salt, baking powder, vanilla extract, milk and egg yolks. Mix together until it forms a cake batter.
5. Add your egg whites and cream of tar tar to a clean bowl and whisk until light and fluffy. Slowly add your sugar a tbsp a time until it is all added, keep whisking until you are able to hold the bowl upside down without the whites falling out.
6. Now add a 1/4 of the egg whites to your cake mixture and fold in, DO NOT WHISK! This will knock all the air out of your whites and the cake wont rise, add a quarter at a time and repeat this process until all has been added. Do not over mix, as long as you can no longer see the egg whites then it is done.
7. Add half the mixture to one tin and the other half to another. Place in the oven for 30-40 minutes until risen and brown.
8. Your cake should be lovely and brown, leave to cool and then remove and put on the side.
9. While cooling beat your butter until smooth, add your vanilla and half your icing sugar until mixed through. Then add the rest of the icing sugar, if you find it thickening add the milk until it is of a smooth consistency.
This is my trusted recipe that I used whilst running my cake business, my customers loved it and I hope you do to.