Combine dry ingredients. In a mixing bowl, whisk all-purpose flour, sugar, baking powder, and salt and whisk to combine. Set aside. 1 cup (200 g) granulated sugar 2 cups (240 g) all-purpose flour 1 Tablespoon baking powder ½ teaspoon salt
Combine wet ingredients. In a second mixing bowl, whisk together egg ,milk, melted butter, and vanilla extract. 1 large egg ⅔ cup 160 ml milk, ½ cup (113 g, 1 stick) unsalted butter 2 teaspoons vanilla extract
Combine wet and dry ingredients. Add the dry ingredients to the wet ingredients and mix together with a spatula or wooden spoon just until just combined. Do not overmix. Let the batter rest for 15 minutes. While the batter rests, preheat the oven and prepare the pan.
Preheat and prepare pan. Position a rack in the center of your oven and preheat the oven to 425F. Line a muffin pan with paper liners. I line two muffin pans, and line every other well (this helps for tall muffins).
Add streusel. Remove the streusel from the freezer and crumble into smaller pieces with your fingers. Fold the streusel into the batter. Divide muffin batter into the muffin wells up to the top.
Bake the muffins. Bake at 425°F for 5 minutes, then, keeping the muffins in the oven (do not open the oven door!), reduce the oven temperature to 350°F (177°C) and continue to bake for 16-20 minutes or until the tops are lightly golden brown and centers are set. Stick a toothpick in the center of a muffin to test for doneness. If it comes out clean or with a few moist crumbs the muffins are done. Allow to cool for 10 minutes in the muffin tray then remove and allow to cool on a wire rack.