Preheat the oven to 375°. Line one 12-cup muffin tins with paper liners or spray with nonstick cooking spray.
In a large mixing bowl, combine the milk, oil, yogurt, eggs, and vanilla.
In another large bowl, whisk together the flour, sugar, baking powder, and salt. (Or use this homemade muffin mix!) Fold the dry ingredients into the wet. Add the chocolate chips and gently fold.
Divide half the batter into the prepared cups. Spoon about 1 tablespoon jam over the batter and then top with the remaining batter. Sprinkle the tops with a generous topping of demerara sugar.
Bake until golden brown and a tester inserted comes out with a few crumbs attached, about 20 minutes. Cool on a rack.
Notes
To freeze: cooled muffins can be stored in an airtight container in the freezer for up to 2 months.