Preheat oven to 350°. Grease 10 cavities of a standard muffin pan.
Stir together the flour, sugar, baking powder, and salt. Make a well in the center of the mixture.
In a separate bowl, mix the milk, oil, egg, and vanilla. Add to the flour mixture, and stir just until combined.
Fill each muffin cup about 2/3 full with batter, leaving about 3 tablespoons of batter in the bowl.
Place about 1/2 teaspoon of jam in the center of each filled muffin cup. Top with just enough of the reserved batter to cover the jam.
Bake for 18 to 20 minutes, or until the edges are browned and the muffins are set. Set aside to cool slightly.
To make the topping:
Combine the sugar and cinnamon in a shallow bowl. Brush the melted butter over the tops of each muffin. Dip each muffin, top-side down, into the cinnamon-sugar to coat.