In a small bowl, mix together the streusel ingredients by using your fingertips to blend the flour, oats, sugar and butter together until it resembles coarse crumbs. Set aside.
Preheat the oven to 400 degrees. Line a muffin tin with paper liners or spray liberally with cooking spray. In a large bowl combine the flour, sugar, baking powder, salt and cinnamon. Make a well in the center of the flour mixture and mix together the milk, butter, egg and vanilla in the well before incorporating it with the flour. Mix the batter together until everything is moist (the batter will be lumpy).
Fill each muffin liner halfway with batter making a divot with the back of a spoon in the middle of each, then place a teaspoon of jam in the center of each. Fill each muffin liner the rest of the way with the remaining batter. Top each muffin with a spoonful of streusel topping.
Bake for 20-25 minutes or until the tops of the muffins are golden brown and a toothpick comes out clean. Cool on a wire rack.
Tip: Use your favorite jam, jelly or preserves as the filling!