Mix biscuit crumbs with melted butter. Toss biscuit crumbs into a lined 18cm cake pan and press firmly. Bake biscuit base at 160C for 12 minutes. Cool for use later.
Cream cheese with sugar till smooth and creamy.
Add in egg one at at time and beat till well blended.
Using low speed, beat in the sour cream and milk.
Divide batter into 2 equal portions of about 490g per portion.
Stir melted chocolate and rum into 1 portion of batter till well blended.
Add lemon juice into the other portion of the batter.
Spoon 1 ladle full of plain batter into the middle of the biscuit base, then 1 ladle full of the chocolate batter on top of the plain batter. Alternate between the two batters till all batter is used up.
Bake at 160C for 50 - 60 minutes till the circumference of the cake is cooked. The centre of the cake could be wobbly. Remove cake and cool for 1 hour. Chilled cake for at least 3 hours before serving, or overnight for better flavour.