Put the ricotta, milk, sugar and whole eggs in a bowl and mix well with a spoon.
Add the sieved potato starch and flour little by little and mix all the mixture using the spoon.
Add the baking powder and mix well the dough that will result quite firm.
Wash the grapes and cut them in half or smaller pieces and remove the seeds if any. Put more than half of the grapes in the muffin dough and mix well, and leave a few for decoration.
Grease the muffin tray and pour the mixture of the grape muffins.
Decorate the muffins with the remaining grapes. Sprinkle them with brown sugar.
Cook the muffins in a preheated ventilated oven at 180° for about 25 minutes until the muffins are cooked evenly, always do the toothpick test.
Take the muffins out of the oven and let them cool completely before serving.