Line a muffin tin with 12 standard sized paper liners and preheat oven to 350.
Mix together flour, baking powder and salt.
In a separate bowl, mix together brown sugar, nuts and cinnamon.
Put butter in a mixer and beat on high speed until fluffy, about 30 seconds. Reduce speed to medium and slowly pour in sugar, scraping down the sides of the mixer as needed. Beat for about 2 minutes.
Add egg, beating well, followed by vanilla.
Reduce speed to low and alternately add flour mixture and milk in about 3 stages.
Spoon one tablespoon batter into each muffin cup. Top with a generous teaspoon of cinnamon mixture. Spoon remaining batter on top of each muffin, and then sprinkle with a generous topping of cinnamon mixture.
Bake for 15-20 minutes until a toothpick inserted in the center of a muffin comes out clean. Cool in the muffin tin until the muffins are cool enough to take out, and then remove and finish cooling on a rack.
Mix sugar, vanilla and milk in a small bowl until smooth – use one tablespoon of milk to start, and then drizzle in more if you want a thinner consistency.
Drizzle glaze over top of muffins (or you can make your own disposable pastry bag by filling a zip-close sandwich bag with the glaze, snipping off one corner with a scissors and squeezing the glaze out through the opening).