In a small bowl, mix together filling ingredients and set aside.
3 tablespoons butter, ½ cup brown sugar, 2 teaspoons ground cinnamon
In a separate bowl, add the streusel ingredients. With clean hands, mix together, squeezing to form clumps until all ingredients are incorporated in clumps. Set aside until ready to top the muffins
¼ cup rolled oats, ½ teaspoon ground cinnamon, ⅓ cup brown sugar, 3 tablespoons unsalted butter, ½ cup all-purpose flour
Preheat the oven to 425ºF. Line a 6-cup jumbo muffin pan with liners or spray with nonstick spray and set aside. If making standard sized muffins, line a 12-cup muffin tin with paper cupcake liners or spray with nonstick cooking spray. Set aside
In a large mixing bowl, whisk together flour, baking powder, salt and cinnamon. Set aside.
2 ½ cups all-purpose flour, 1 tablespoon baking powder, 1 teaspoon kosher salt, 2 teaspoons ground cinnamon
In a separate large bowl, mix together with a wooden spoon the vegetable oil, brown sugar, eggs, milk and sour cream until well incorporated.
⅓ cup vegetable oil, 1 cup brown sugar, 2 large eggs, 2 teaspoons vanilla extract, ½ cup whole milk, ½ cup full-fat sour cream
Gently fold in the dry ingredient mixture until no dry spots of flour are left.
If using a jumbo muffin tin, add ¼ cup muffin batter to the bottom of each muffin cup. If using a standard muffin tin, add 2 tablespoons of muffin batter. Add 2 teaspoons of the filling for jumbo, 1 teaspoon for standard, then top with ¼ cup more muffin batter (2 tablespoons for standard)
Sprinkle muffin batter generously with streusel topping, dividing it evenly among the muffins
Bake for 5 minutes at 425ºF, then lower the oven temperature down to 350ºF for another 15 minutes for standard muffins and 18-20 minutes for jumbo, or until a toothpick inserted into the center of the center muffin comes out clean.
Allow to cool in muffin tins for 10 minutes, then transfer to a wire rack to cool completely.
In a small bowl, mix glaze ingredients together and drizzle on top prior to serving
⅔ cup confectioners' sugar, pinch salt, 1 tablespoon milk, 1 teaspoon vanilla extract