Add lemon juice (or white vinegar) to the heavy cream and set aside to make 'buttermilk'.
Preheat the oven to 425°. Spray the donut molds/pans with a non-stick spray
In a mixing bowl, combine the flour, baking powder, cinnamon, nutmeg, and salt.
Add the melted butter and stir thoroughly before also adding the heavy cream and beaten eggs. The dough will be a very sticky consistency.
Gentle fold-in the drained blackberries.
Fill each of the donut ring cavities, making sure it does not cover the center hole. This can be tricky. Use a spoon or even clean hands
Bake about 10-12 minutes until the tops rebound when pressed. Remove from the oven.
Leave the donuts in the molds/pan for about 5 more minutes before popping them out of the pan and onto a cooling pan.
You can serve the donuts now....or apply a white glaze.
To make the glaze, mix the powdered sugar, heavy cream and, vanilla into a small bowl just large enough to dip the top of the donuts. Hold the dipped donut above the bowl to allow the excess glaze to drip off.
Return the glazed donuts to the cooling rack to allow the glaze to set before serving.