Preheat the oven to 150 °C and grease a 20cm loose based tin.
Melt the butter and stir in the biscuits and half of the crushed Crunchie. Mix well and then press into the base of the tin. Set aside.
In a bowl, beat the cream cheese and sugar together then add the flour. Mix well then beat in the eggs and soured cream until smooth. Pour 1/3 of the batter into a small bowl and mix in the cocoa.
Pour the vanilla batter over the biscuit base and then swirl the chocolate into it.
Bake for 45 minutes, turn off the oven and allow to cool with the door slightly ajar for 1 hour. Remove and chill for a couple of hours or overnight. Before serving, remove from the tin and place onto a serving plate and scatter the remaining 2 Crunchies over the top.