Line a 20 x 30cm baking tin with baking parchment. Heat the oven to 180C/160C fan/gas 4.
Mix the sugar, egg whites, vanilla and a pinch of salt together in a bowl. Add the coconut and flour and stir until thoroughly combined – the mixture will be quite thick and sticky. Tip into the tin and gently spread out with a spatula. Dot the jam over the top using a teaspoon, nestling it into the batter. Bake for 25-30 mins, until set and golden around the edges. Leave to cool completely in the tin.
Melt the chocolate in the microwave, or in a bowl set over a pan of simmering water. Remove the cake from the tin, drizzle with chocolate and leave to set before cutting into squares. Will keep in an airtight container for up to a week.