Make the cake batter first. Preheat your oven to 180 degrees C (fan). Grease and line three 8 inch / 20 cm cake tins.
Cream the butter with the caster sugar until it is soft and no longer grainy.
Add the eggs one by one, and again mix well.
Add the vanilla bean paste to the milk, stir, and then add some of the milk, followed by some of the flour, to the mixture, and combine as you go.
Divide the cake mixture equally into three 8 inch / 20 cm tins.If you only have two tins in the same size, you can bake two layers first then the third after the first two are baked. It is best to line the tin in this case so it is easy to take the first layer out and refill the tin. Bake for approx. 20 mins, or until the cakes shrink away from the edge of the tins. Check using a cake tester to ensure the layers are cooked through. Allow to cool for a few minutes, then remove from the tins and leave the layers to cool completely. While the cakes are baking make the buttercream icing.Put the softened butter in a stand mixer and beat for around five minutes until it lightens in colour and is soft and fluffy. Add the icing sugar in batches of about a teacup full at a time, mixing well each time you add more.A top tip for making buttercream is to turn the mixer speed down when you add the icing sugar, to lessen the clouds of icing sugar. If you have a mixer with a cover this is a good time to use it.Once each cupful of icing sugar is completely incorporated and the mixture looks light and fluffy again, you can add the next cupful. Finally, once all the icing sugar is added, continue to mix and add a teaspoon of milk and beat again.Add your gel colour, if using, and mix until the icing reaches your desired colour. You may need to add more to achieve a deeper colour.
One the cakes are completely cool, take the first cake and top it with buttercream, add the next layer and top with buttercream, then add the final layer and top with buttercream.
Add buttercream around the edges of the cake using a palette knife and smooth round roughly. Put the cake aside for a couple of hours for this crumb coat to set.
Once the crumb coat is set, you can do the final decoration on your birthday cake.Add another layer of buttercream on top of the crumb coat, around the sides and on top.
Use a cake smoother to make the edges straight and smooth off the sides. Decorate the top with sprinkles, or chocolate beans as I used, as you wish.