Preheat the oven to 175°C
In a saucepan, combine the brown sugar, golden syrup and butter. Heat while stirring until all the sugar has dissolved and the mixture starts to boil (roughly 5mins). Set aside and allow to cool for 15 mins.
Once cooled, whisk in the eggs, vanilla, bourbon and nuts, then pour into the prepared tart shell. Bake for 35mins.
Meanwhile make the bourbon mascarpone by mixing together the mascarpone, icing sugar and bourbon.
Once the tart has cooled, remove it from the mould and cut into even pieces. Serve with a large dollop of mascarpone.