In a large bowl, whisk together the flour, granulated sugar, baking powder and salt.
In a medium bowl, whisk together milk, butter and egg. Stir into the flour mixture until just combined. Do not overmix.
Grease a 12-cup muffin tin and put a scoop of the apple/grape mixture in each cup. Divide the batter evenly among the muffin cups. Bake for 15-18 minutes, until golden and a toothpick inserted into the center comes out clean.
Let cool on a rack for about 2 minutes, then run a knife around the edges and invert onto a rack to cool completely. Dust with confectioners' sugar before serving, or for dessert, serve with whipped cream.
Recipe by Hidden Ponies at https://hiddenponies.com/2013/10/apple-grape-upside-down-muffins/