Hi Liz! Hi there!

Oh, yummyscrumbums, treacle tart!

Definitely golden syrup, as eaten by most Brits, although the Scots might disagree
Treacle itself (black treacle) has a very different taste, like black molasses (strong, smoky, liquorce-ish) and and wouldn't really be interchangeable for some things, e.g. ginger bread - also very scrummy.

Found my recipe, it produces a pretty definitive treacle tart as recognized by most Brits,
all the best


Treacle Tart
Serves: 4-6
Ingredients

110g plain flour
25g lard
25g butter
8 tablespoons golden syrup
125g fresh white breadcrumbs
Grated zest of 2 lemons, plus 2 teaspoons of lemon juice

Instructions
Sieve the flour into bowl. Add the fats and rub into the flour until it resembles fine breadcrumbs. Add a small amount of water and mix to a firm dough. On a floured board, thinly roll out the pastry and use to line an 18cm flan dish. Any excess pastry can be rolled out again and used to decorate the top after the filling is added. Allow the pastry to rest.

Mix together the syrup, breadcrumbs, lemon zest and juice. Spoon the filling into the pastry base, decorate as required, and bake for 25 minutes at 180°C, gas mark 4 until the pastry is golden brown. Serve warm or cold.