Prepare your cake pans. Line 2 to 3 round cake pans with paper rounds and coat and grease with butter. Arrange a rack in the middle of the oven and heat to 180°C/fan 160°C/gas mark 4.
Cream the butter and sugar. Place the butter and sugar in stand mixer fitted with a paddle attachment and beat on medium speed until the mixture is lightened in colour and slightly increased in volume, about 5 minutes.
Add the egg yolks. Stop and scrape down the sides of the mixer with a rubber spatula. Return the mixer to low speed and add the egg yolks one at a time, allowing each to fully incorporate into the batter before adding the next.
Add the Flour. Stop and scrape down the sides of the mixer with a rubber spatula. Return the mixer to low speed and add the flour a bit at a time.
Add the vanilla and milk.
Mix for 3 minutes on medium speed. Stop the mixer and give the whole bowl another good scrape. Get down to the bottom of the bowl where butter and sugar tend to hide and will make your cakes colour and bake unevenly. Return the mixer to medium speed and b
Divide and colour the batter. You’ll be baking 2 or 3 cakes at a time, depending on the number of cake pans you have. Weigh and divide the batter equally between 6 bowls.
Add 1 teaspoon of gel food colouring and mix well. Pour the coloured batter into a prepared cake pan. Repeat with your other colours.
Bake the 2 to 3 coloured cakes you have prepared for around 15-20 minutes until they are set and spring back when touched.
Remove the cakes and put on a cooling rack and cool. Remove the cakes from the pans, wrap each still slightly-warm cake separately in cling film whilst you continue with your other layers.
Repeat dividing the batter, colouring it, and baking. Cool and wrap the remaining cakes. Set aside at room temperature.
Beat the butter in the bowl of a stand mixer fitted with a paddle attachment on medium speed until smooth.
Turn the mixer speed to low and slowly add in the sugar. Once it’s all added in, increase the speed to medium-low and mix until thoroughly combined.
Reduce the mixer to low, add in the cream or milk and vanilla, and mix until combined.
Mix on medium speed until smooth and creamy.
Decide what order you wish to have your cake layers, the middle two layers are the ones that will need to have a hole for the sweet fillings. You may wish to level your cake layers if they are too tall or uneven.
Take your chosen two middle layers and press a hole in the centre of the cake with the cookie cutter.
Put a blob of buttercream in the centre of a cake stand. Put your first cake layer central on the stand. Top with some buttercream and spread until level. Place your second cake layer on top of the first as central as possible. Again, top with some butt
Pour the sweets into the hole, and fill until level with the top cake layer.
Place the 5th layer on top of the cake, topping with an even level of buttercream. Place the final layer on top of the cake and crumb coat the whole of the cake with a thin layer of buttercream.
Chill the cake in the fridge for at least an hour.
Cover the whole cake with the remaining buttercream until you have your desired smoothness. Decorate however you wish, meringue kisses make for a wonderful topper, as well as sprinkles and your favourite colourful sweets.