1 Preheat oven to 425 degrees. Spray a standard muffin tin with vegetable oil spray (or line with paper liners and spray).
For the blueberry jam:
3 Bring blueberries and 1 teaspoon of the sugar to a simmer in a small saucepan over medium heat. Cook, mashing the berries with a spoon several times and stirring frequently, until the berries have broken down and the mixture is thickened about 6 minutes. Transfer to a small bowl and cool to room temperature, 10-15 minutes.
For the muffins:
5 Whisk the flour, baking powder and salt together in a large bowl. Set aside.
6 In a medium bowl, whisk together 1-1/8 cups sugar and eggs until thick and well combined, about 45 seconds. Slowly whisk in the butter and oil until combined. Nest, whisk in the milk and vanilla until combined.
7 Using a rubber spatula, fold the egg mixture into the flour mixture until just moistened. The batter will be very lumpy with a few spots of dry flour; do not overmix. (If using whole blueberries, fold in now.)
8 Fill muffin cups 1/2 full with batter and top each muffin with a teaspoon of the blueberry jam. Use a toothpick to swirl. Divide the remaining batter between muffin cups. The batter should completely fill the cups and mound slightly. Divide the remaining cooked berry mixture between muffin cups. Use a toothpick to swirl.
Make the streusel topping:
10 To make the crumb topping in a small bowl, whisk together flour, sugar and cinnamon, add melted butter and stir with a fork until crumbly. Spoon streusel over muffins.
11 Bake until the muffin tops are golden and just firm, 17-18 minutes, rotating the muffin tin halfway through. Cool the muffins in the muffin tin for 5 minutes, then remove to a wire rack and cool 5 more minutes before serving.
