Crush the digestive biscuits in a food bag with a rolling pin or in the food processor. Melt the butter in a saucepan, take off heat and stir in the brown sugar and biscuit crumbs.
Line the base of a 20cm loose bottomed cake tin with baking parchment. Press the biscuit into the bottom of the tin and chill in the fridge while making the topping.
Beat together the mascarpone, caster sugar, lemon zest and juice, until smooth and creamy. Spread over the base and chill for a couple of hours.
