Sift flour and baking powder into a large bowl and mix in 125g sugar and a pinch of salt. Make a well in the centre and add melted butter, stirring with a knife to gradually draw in the flour. Add eggs one at a time, stirring until mixed. Shape into a ball (if too soft, add an extra tablespoon of sifted flour), wrap in plastic film and refrigerate for one hour to help firm it up. Heat the oven to 180C. Use a floured rolling pin to lightly roll out the dough or just pat it out with floured hands into a rough oval shape on a sheet of baking paper, then transfer, on the paper, to a baking tray. De-stem the grapes and cut in half lengthways. Scatter the grapes on top, half of them cut-side up, half cut-side down, pressing in lightly. Scatter with remaining sugar and bake for 15 minutes until golden, when a thin skewer inserted into the middle should come out clean. Cool and serve lightly warm or at room temperature. Best eaten on the same day as baking.