1. Heat oven to 180℃/160℃ fan/gas 4. Melt the white chocolate, milk, butter and Irish Cream liqueur in a heatproof bowl over simmering water until the mixture is creamy and smooth. Whisk in the sugar, vanilla and a pinch of salt, then remove the bowl from the heat and set aside to cool.
2. Once the chocolate mixture has cooled, whisk in the eggs, stir in the sifted flour, nutmeg and baking powder until just combined. Spoon the mixture evenly between 2 round greased and lined 20cm cake tins.
3. Bake the cakes for 30-40 minutes until golden, cool for 10 minutes in their tins and then turn out onto a wire rack.
4. To make the buttercream, beat together the softened butter, vanilla and icing sugar until pale and fluffy and decorate your cake. 

The cakes can be baked in advance and frozen for up to 3 months. Wrap them well before freezing and defrost at room temperature before decorating.