1Heat the oven to 350°F and arrange a rack in the middle. Line a 12-well (1/2 cup) muffin pan with paper liners; set aside.
2Sift the flour, baking powder, baking soda, nutmeg, and salt into a large bowl. Make a well in the center of the mixture; set aside.
3In a medium, nonreactive bowl, whisk together the soy or rice milk, vinegar, and cornstarch until the cornstarch has dissolved. Pour this into the well in the flour mixture. Add the granulated sugar, oil, and vanilla and stir with a rubber spatula until combined (there will be a few lumps).
4Fill each muffin well about three-quarters full. Using a spoon, create a small indentation in the batter by slightly spreading it from the middle out toward the edges. Measure 1 heaping teaspoon of jam or preserves and place the back of the teaspoon inside the indentation. Rotate the spoon, letting the jam slide into the indentation. Repeat in each well.
5Bake until the tops of the muffins are firm, about 21 to 23 minutes. Set the pan on a wire rack and let cool for 5 minutes. Remove the muffins from the pan and let cool completely on the wire rack. Dust with powdered sugar before serving.