Preheat your oven to 425°F (218°C). Line a 12-cup muffin tin with paper liners or grease the cups generously with non-stick spray.
Spread the chopped pecans on a baking sheet and toast in the preheating oven for 5 minutes until fragrant. Remove and let cool.
In a large mixing bowl, whisk together the flour, brown sugar, baking powder, salt, and cinnamon until well combined and no lumps remain.
In a separate medium bowl, whisk the eggs, milk, melted butter, and vanilla extract until smooth and fully emulsified.
Pour the wet ingredients into the dry ingredients. Mix with a spatula or wooden spoon gently until just combined. Do not overmix; a few streaks of flour are
Gently fold in the toasted pecans until evenly distributed throughout the batter.
Divide the batter evenly among the 12 muffin cups, filling them all the way to the top for a bakery-style dome.
Bake at 425°F (218°C) for 5 minutes. Then, keeping the oven door closed, reduce the temperature to 350°F (175°C) and bake for another 13-15 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
While muffins cool, whisk the powdered sugar, vanilla, and 2 tablespoons of milk in a small bowl. Add more milk one teaspoon at a time until the glaze reaches a drizzle-able consistency.
Drizzle the glaze generously over the cooled muffins before serving.