Make the curd the night before. Heat the milk in a large pan. As it comes to a gentle simmer, add the lemon juice. Turn the heat to low and gently stir while the curds form. Do not stir too quickly or you will break up the curds. Once the mixture resembles watery liquid with creamy lumps in it, remove the pan from the heat and allow the curds to cool in the way. Drain the curds overnight through a clean tea towel and save the whey for making scones, as you would buttermilk.
To make the pastry, blend the flour, baking powder, butter, sugar and a pinch of salt in a food processor until the butter is almost all combined – leave a few small lumps of butter to lighten the dough. Tip the dry mix onto a work surface and make a well in the centre. Add a little cold water to make a smooth, but not sticky dough. As soon as the mixture comes together, knead very lightly, wrap in cling film and chill for at least 20 mins. Can be made up to 2 days ahead and refrigerated.
To finish the filling, beat the butter and sugar until soft, then add the egg, a little bit at a time. Add the curds to the mixture and lightly whisk to break up any large lumps. Once blended, add the currants.
Heat oven to 180C/160C fan/gas 4. Roll out the pastry to line a greased 20cm shallow pie dish or tart tin. Spread over the curd mix and bake for 35-40 mins until browned and cook the cake. Leave to cool, then cut into slices and serve on its own or with a drizzle of cream.