Heat the butter and milk together until almost boiling, either in a microwave (30 secs at a time) or in a pan over the stove on a medium heat.
Break the chocolate up into small pieces in a heatproof bowl and pour the hot butter and milk mix over the chocolate. Cover and leave for 3-4mins.
Stir the mix until smooth, with all of the chocolate melted. If the chocolate hasn’t fully melted (i.e. you can see small chocolate lumps) you can put this back into the microwave / over the heat for a bit longer until smooth.
Add in the lemon zest and juice, stirring until evenly mixed throughout.
Let the mix cool until set for at least an hour in the fridge, then use as desired.
This recipe makes enough ganache to frost a 2-layer cake (8-inch sponges, icing between layers and covering the outside) or roughly 14 cupcakes.
Store your ganache in an air-tight container in the fridge for up to 4 days.