Photograph: Yuki Sugiura
Preheat the oven to 180°C, fan 160°C, gas 4. Grease a 20cm-diameter cake tin. Melt 50g of the butter in a small saucepan, add the molasses or sugar and stir until dissolved. Gently simmer until rich and dark, then tip into the cake tin. Arrange the apple slices on top.
Beat the light brown sugar and the remaining butter together until light and fluffy, then beat in the eggs, a little at a time. Fold in the flour, spices and salt, and spread over the apples. Bake for 35-40 minutes, until springy to the touch. Leave to cool in the tin.
For the boozy currants, mix the caster sugar and rum in a small pan, add the currants and simmer gently for 10 minutes, then set aside.
To serve, gently warm the cake in a low oven, if you fancy, though it's just as good served cold. Warm the currants over a gentle heat. Turn the cake out on to a plate, slice and serve with a blob of crème fraîche and a spoonful of currants.
Get ahead: make up to the end of step 3 up to 3 days ahead.