Preheat the oven to 425°F / 220°C. Alternate lining a muffin pan with 6 parchment liners.
Chai Streusel
In a small bowl, melt the butter. Use a fork to whisk in the sugar, flour, and chai spice until combined and chunks of dough are formed. Set aside.
Coffee Muffins
In a medium sized mixing bowl, melt the butter, then whisk in the sugar.
Whisk in the egg, vanilla, coffee, sour cream, and milk until well combined.
Add in the flour, baking powder, and salt. Gently mix together just until no flour streaks remain.
Use a small cookie scoop to dish out enough muffin batter into each liner to cover the bottom (~1 ½ tbsp.)
In a small bowl, whisk together the brown sugar and chai spice. Evenly distribute the spiced sugar between each muffin liner, using approximately 2 tsp. per muffin.
Top the muffins with the remaining batter, then evenly distribute the streusel on top of each muffin.
Bake the muffins for 5 minutes at 425°F / 220°C. Without opening the oven door, reduce the temperature of the oven to 350°F / 175°C. Bake for an additional 10-15 minutes, or until a toothpick inserted into the muffins comes out clean.
Coffee Glaze
In a small bowl, whisk together the powdered sugar and 1 tbsp. of coffee until well combined. If the glaze is too thick, add in more coffee until the desired consistency is reached.
Once the muffins have cooled to room temperature, drizzle the glaze over each muffin. Serve and enjoy!