Pre-heat oven: 180°C / 160°C (fan) / Gas Mark 4 / 350°F
Cream together the butter and sugar until smooth. This can be done in an electric mixer if you have one.
Beat the eggs into the mix one at a time until fully combined.
Sift in the flour and add the lemon zest, folding these in using a spatula or wooden spoon, then stir in the milk until only just combined.
Split the cake mix evenly between the 3 lined tins and bake for 20-25mins, until golden and a skewer comes out clean from the centre.
While the cake is baking, start making the ganache.
Heat the butter and milk until almost boiling, either in the microwave or in a saucepan.
Break up the chocolate into small pieces in a heatproof bowl and pour the hot butter and milk mix over the chocolate. Cover and leave for 5mins.
Stir the ganache until smooth. If all of the chocolate hasn't melted, re-heat for 10secs in the microwave or 2-3mins on the hob and stir again. Repeat this until completely melted and smooth.
Add the lemon zest and juice, and stir until combined.
Let the ganache cool for 30mins-1hr in the fridge until set. It should be thick and spreadable once set and firmed up.
Assembly and Decoration:
Let the cakes cool completely before slicing off any peaks on top to a level sponge.
Add the generous amount ganache between each of the layers, spreading evely, and on top. If you'd like, you can also cover the outside of the cake (see notes below for tips).
Decorate with lemon slices and grated lemon zest.
If you are after a more professional finish, you can coat your cake in a crumb coating (a thin layer of the ganache) first, after assembling the cake, and refrigerate for 15mins. After this, fully cover the outside of the cake in ganache and smooth with a scraper.
If you have used 2 cake tins rather than 3, bake your cakes for an extra 5-10mins and cut these sponges in half to get 4 layers. You can use toothpicks to help guide you into cutting a level slice, or use a cake leveller if you have one.