Preheat oven to 350°F/180°C. Lightly grease a 12-cup muffin pan and set aside.
Combine flour, brown sugar, salt and cinnamon in a medium bowl and rub together to push out any lumps of sugar. Add cold butter and rub it in until well blended and mixture is crumbly. Divide mixture evenly among wells the prepared muffin pan. Press it in firmly and evenly.
Bake for 10-12 minutes, until browned and fragrant. It should smell like spiced toffee. Transfer pan to a wire rack to cool. Reduce oven temperature to 300°F/150°C.
To make the filling, beat cream cheese until smooth. Add brown sugar and vanilla extract and beat until very smooth and glossy. Beat in egg until blended. Add still warm melted chocolate and stir it in until incorporated. The mixture will be thick.
Spoon mixture over cooled crusts, dividing it evenly. Spoon a little melted white chocolate over the filling and swirl it in with a skewer to create a marbled effect. Bake at 300°F/150°C for 10-12 minutes until edges are set but filling is still a bit soft.