This is an early Jamie Oliver recipe, from The Naked Chef, which I think was published in 1999. It's fairly heavy on sugar, but doesn't taste too sweet because of the large amount of cocoa in the chocolate filling. Adding golden syrup helps the filling bake to a rich fudgy texture, similar to a very rich brownie.
You need a pre-baked shortcrust pastry tart shell, a maximum of 25cm(10") in diameter. I make my own sweet shortcrust pastry and use a 23cm (9") fluted shallow tart tin, but a bought pastry case is OK too.
140g unsalted butter
150g dark chocolate (70% cocoa solids)
8 tablespoons cocoa
4 large eggs
200g caster sugar
3 tablespoons golden syrup
3 heaped tablespoons sour cream/creme fraiche
Preheat oven to 150C or 130C fan
Melt the butter, chocolate, cocoa and salt together in a bowl over a pan of simmering water. Stir to mix thoroughly. Remove from heat.
In a separate large bowl, mix the eggs and sugar until thick and creamy, then stir in the golden syrup and cream.
(If you've used a case which is less than 25cm in diameter you will have some chocolate mixture left over, but this can be poured into two or three shallow ramekins and baked alongside the tart.)
Bake for 35 - 45 minutes until the mixture feels firm in the centre. My 23cm tart cooked in 35 minutes, but the recipe suggests a bit longer for the bigger size.
Cool for at least 45 minutes before serving. I serve at room temperature, dusted with icing sugar. There's no need to refrigerate any leftovers.