Cream butter and sugar until light and fluffy.

Beat in molasses, egg and vanilla extract until smooth.

In a separate bowl, mix flour, spices, baking soda and salt.

Fold dry ingredients into the wet until just combined.

Spread batter evenly in the prepared tin.

Bake for 25-30 minutes until edges are set and the centre is slightly soft.

Cool completely before drizzling with white chocolate and slicing into bars.