First, make the pastry. Rub the butter into the flour and icing sugar until it is the consistency of fine breadcrumbs. Add the egg yolks, and mix to combine. Wrap the pastry and then allow to rest in the fridge. This could be made ahead of time and kept in the fridge for several days. If the dough is very firm, allow to soften slightly at room temperature before rolling out.
Line 12 small tart cases or 1 large 23cm tart case and allow to rest in the fridge or freezer before baking.
Next, make the filling. Weigh out the syrup and place into a medium sized saucepan along with the bitter. Warm gently until the butter has melted, do not allow the mixture to boil.
In a large bowl, mix the egg, cream and lemon zest until combined.
Add the syrup and butter to the eggs and cream and mix together. Add the breadcrumbs, keeping 2 handfuls in reserve. Stir to combine and allow to stand for 5 minutes to allow the breadcrumbs to swell slightly.
Spoon the mixture in to the prepared cases until it is just below the rim of the pastry. Even the mixture out, if necessary.
Place onto a baking tray and bake for 25 / 30 minutes on 170C.
Remove from the oven once the treacle mixture is just set and the tops are golden.
Leave to cool in the tins. Serve hot or cold with custard, cream or ice-cream.