In a small bowl, combine 3/4 cup of all-purpose flour, 1/2 cup of granulated sugar, and 1/2 teaspoon of ground cinnamon. Add 6 tablespoons of chilled unsalted butter cut into tiny pieces. Using a fork or your fingers, mix until the mixture resembles coarse crumbs. Set aside.
In a large mixing bowl, whisk together 3 cups plus 2 tablespoons of all-purpose flour, 1 tablespoon of baking powder, 1/4 teaspoon of ground nutmeg, 1/2 teaspoon of ground cinnamon, and 1/2 teaspoon of salt. Make sure everything is well combined.
In another bowl, beat together 6 tablespoons of room temperature unsalted butter, 1 cup of granulated sugar, and 1/3 cup of light brown sugar until the mixture is light and fluffy, about 2-3 minutes.
Add 1 large egg and 2 large egg yolks to the butter mixture, one at a time, mixing well after each addition. Stir in 2 teaspoons of pure vanilla extract until combined.
In a separate bowl, combine 3/4 cup of whole milk and 1/3 cup of full-fat sour cream. Gradually pour this mixture into the wet ingredients, mixing gently until just combined.
Slowly add the dry ingredient mixture to the wet mixture, folding with a rubber spatula until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in 1 and 1/2 cups of blueberries, taking care not to crush them.
Preheat your oven to 375°F (190°C). Line your muffin tin with parchment paper or muffin liners. Spoon the batter evenly into each muffin cup, filling them about 3/4 full. Top each muffin with the crumb topping you prepared earlier.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean. The tops should be golden brown and slightly crisp.
Allow the muffins to cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely. Enjoy your Jumbo Blueberry Crumb Muffins warm or at room temperature!
