Grease a 20cm round cake pan (square is also fine) and line the base and sides of the pan with baking paper - or use a spring from pan - grease with butter or oil.
Melt butter and sugar in a large saucepan over low heat, stirring occasionally. Add the white chocolate and stir until melted. Add - a small amount at a time - the milk and White Heaven Crème Liqueur, stir until combined. Allow the mixture to cool at room temperature for 15 minutes.
Add the Butterscotch Liqueur and lightly beaten eggs to the cooled chocolate mixture and stir until well combined.
Sift the flours together in a large bowl. Add one cup of the chocolate mixture to the flour and stir until a smooth paste forms. Repeat with another cup of the chocolate mixture.
Add the remaining chocolate mixture and stir until the mixture is smooth. This gradual method of combining the wet and dry ingredients helps prevent lumps.
Pour the mixture into the prepared cake pan.
Bake for about 1 hour and 10 minutes. (When the cake is ready, a fine-bladed knife inserted into the centre of the cake should come out without any batter attached.)
Loosely cover the cake with greaseproof paper or a clean tea towel and allow to cool to room temperature in the pan.
The cooled cake can be iced with the white chocolate ganache immediately (refer recipes) or stored and iced on the day of serving. (Store in an airtight container in the refrigerator and bring to room temperature before serving.)