Preheat the oven to 350 degrees and line cupcake/muffin tin with paper liners.
Prepare the filling. Mix brown sugar, pecans and cinnamon together. Set aside.
In a stand mixer fit with a paddle attachment, combine butter and sugar. Mix on a medium speed for about 2 minutes, or until fully combined.
Add eggs and vanilla extract. Mix until combined.
Add flour, baking powder, baking soda and salt. Mix until just combined.
Slowly fold in the sour cream.
Pour about 1 tablespoon of batter at the bottom of each paper liner. Using a spoon, spread the batter to completely cover the bottom of the liner.
Sprinkle 1 teaspoon of filling over the muffin batter.
Place another heaping tablespoon of batter into the paper liner over the filling. If necessary, use a spoon so spread batter to cover filling.
Sprinkle another teaspoon of filling on top.
Bake for 18-24 minutes, or until a toothpick comes out clean.
Take the muffins out and set aside to cool.
While the muffins are cooling, make the glaze. Place powdered sugar in a medium bowl. Set aside.
In a small saucepan, melt butter over medium heat. Cook until the butter is slightly brown and gives off a nutty aroma.
Add butter to the bowl of powdered sugar. Make sure you scrape the bottom of the pan and get all the tiny brown bits.
Add vanilla extract and whisk it all together.
Add milk. Whisk until you get desired consistency.
Once muffins are cooled, drizzle the glaze over the muffins.