Turn your oven down to 160°c/140°c fan (320°f/285°f).
Place the panettone into a food processor, blitz into coarse crumbs then set to one side.
Next, place the golden syrup, black treacle, lemon zest & juice and salt into a large saucepan then set over a low heat. Cook for 2-3 minutes, stirring regularly, until the syrup is runny but not hot.
Take the pan off the heat & whisk in the sour cream, followed by the egg & yolk. Switch to a spatula & stir in the panettone crumbs.
Next, spoon the mincemeat into the bottom of the tart case then spread out into an even layer.Make sure to completely cover the base with the mincemeat. Pour the filling into the tart case then bake for 35-40 minutes, until set with a slight wobble.
Once cooked, let the tart cool in the tin to room temperature then place in the fridge to set fully (overnight is best).
To portion, carefully remove the tart from the tin & cut into 10 equal pieces, using a hot, clean knife for each cut (running the knife under hot water & drying off between cuts works best).