Preheat and prep the muffin tins. Preheat the oven to 425°F. Line two 12-cup muffin tins, filling every other cup (6 liners per tin).
Make the streusel. In a small bowl, combine the brown sugar, flour, pumpkin spice, and salt. Stir in the melted butter until fully combined and crumbly. Refrigerate while you prepare the muffin batter.
Mix the dry ingredients. In a bowl, whisk together the flour, baking soda, baking powder, salt, pumpkin spice, and espresso powder. Set aside.
Mix the wet ingredients. In a medium bowl, whisk together the oil, both sugars, and eggs until smooth. Add the pumpkin puree, sour cream, and vanilla extract, and mix to combine.
Combine. Add the dry ingredients to the wet ingredients and mix until just combined; do not overmix.
Fill and top. Using a cookie scoop, divide the batter evenly among the 12 muffin cups. Top each with 1-2 tablespoons of the chilled streusel.
Bake. Bake at 425°F for 5 minutes, then reduce the heat to 350°F (without opening the oven) and bake for another 10-12 minutes, until the muffins spring back to the touch or a toothpick inserted comes out with just a few moist crumbs.
Make the glaze. Combine the milk and butter in a small bowl and microwave for about 30 seconds, until the butter is melted. Let cool slightly, then whisk in the espresso powder until dissolved. Whisk in the powdered sugar and vanilla extract until smooth.
Glaze and serve. Let the muffins cool completely, then drizzle with the espresso glaze.