Preheat oven to 180C/350F. Grease an 11cm x 20cm (4 1/2 inch x 8 inch) (top measurement) loaf pan; line base with baking paper, extending the paper 5cm (2 inches) over the long sides.
Cook butter and treacle in a small saucepan over low heat, stirring, until butter melts and mixture is smooth.
Meanwhile, sift dry ingredients into a large bowl. Stir in combined milk and egg, then treacle mixture. Pour mixture into the pan.
Bake loaf for about 35 minutes. Stand in the pan for 5 minutes; turn onto a wire rack to cool.
Meanwhile, combine all ingredients for lemon mascarpone into a small bowl.
Serve treacle loaf warm with lemon mascarpone.