If, for some reason, your jam is too liquidy, and not becoming thick and reduced enough, it might be because of the type of grape you are using. In this case, after you’ve processed the jam in the blender, return it to the sauce pan, then mix 2 teaspoons of cornstarch with one tablespoon of water, add it to the jam, and bring to a boil. Let jam boil for a couple of minutes until thickened. Remove from the heat and refrigerate until ready to use.
Process walnuts in a small food processor until crushed. You should obtain about 1 cup of crushed nuts. Make sure the nuts are finely ground, so the buttercream can be smooth.
Using a spoon, remove the center of each cupcake. Fill it with the cool down jam. Place the piece of cake you removed back on top of the cupcake, to protect the filling and give a better base to the frosting that we will pipe on top of the cupcakes.
Frost cupcakes with the walnut buttercream. Decorate with grapes, and figs on top.
Leftover jam will keep in the fridge for up to 7 days, in case you have any. Cupcakes will hold up well in the fridge, covered, for up to 4 days.