Add the blackcurrants and sugar to the pan on low heat until the sugar dissolves. Turn up to medium heat and cook for about 5 minutes until the blackcurrants go soft and the mixture is thick and saucy. Set aside.
Sponge Mixture
Grease and line your baking tin/s with baking paper, leaving enough so that it overhangs the baking tray. Heat the oven at 160°c fan/180c/350f/gas 4.
Cream the butter, sugar and lemon zest in with your electric mixer for 3 minutes, until pale and creamy. Add the eggs one at a time whilst mixing.
In a separate bowl, mix the gluten free flour, salt and baking powder. Pour half of this into your egg mixture and mix until combined. Pour in the greek yoghurt, mix again, and then add the remaining flour.
Spoon the batter into your tins and smooth out evenly. Spoon and scatter a few dollops of blackcurrant jam on top of the mixture, then swirl through with a fork or knife.
Bake for 40 minutes or until the sponge is cooked. Leave to cool completely.
For the icing, mix the icing sugar with enough water to make a thick glaze. Drizzle over the cooled cake, then slice and serve.
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Notes
I used one baking tray for a taller sponge, but you can easily spread them out over two small baking trays.