To make the topping, heat the sugar and 4 tablespoons of water in a deep, stainless steel saucepan over a medium heat and stir until the sugar has dissolved. Stop stirring and bring to the boil. Boil until the sugar turns a deep caramel colour. (CAUTION: boiling sugar is extremely hot. Handle very carefully. Use a deep pan to avoid bubbling over.)
Quickly remove from the heat and add knobs of the butter and the orange liqueur. Stir, then pour into the base of a deep, fixed-base 20cm/8in cake tin.
Preheat the oven to 180C/160C Fan/Gas 4.
To make the apricot layer, brush the sides of the tin with the melted butter. Arrange the apricot halves, cut-side up, on top of the caramel. Place any extra apricots in a neat layer over the top.
To make the sponge, place all of the ingredients in a large bowl. Whisk for 1½–2 minutes with an electric hand-held whisk until light and fluffy. Spoon on top of the apricots and level the surface. Bake for about 50–55 minutes, until lightly golden and the sponge is coming away from the sides of the tin.
Leave to stand for about 30 minutes, then loosen the sides of the tin and put a plate on top. Carefully turn the cake upside down and remove the tin.
Spoon any extra caramel sauce from the tin over the top of the pudding. Cut into wedges and serve warm.
