In a small pan, melt butter over medium heat. Stir frequently until golden brown and nutty-smelling (4–6 min). Let cool slightly.
Preheat oven to 375°F (190°C).
Line a muffin pan with 9 liners for tall muffins (or 12 for smaller).
For streusel, mix flour, brown sugar, salt, and melted butter until crumbly. Set aside.
In a bowl, whisk flour, baking powder, baking soda, and salt.
In another bowl, whisk brown butter, maple syrup, eggs, yogurt, milk, and vanilla.
Fold wet into dry until just combined.
Gently fold in blackberries (don’t overmix or they’ll turn the batter purple).
Divide batter into muffin cups, filling nearly to the top for a nice dome.
Sprinkle each generously with streusel.
Bake for 18–22 min until golden and a toothpick comes out clean.
Let cool in pan 5 minutes, then transfer to wire rack.
Whisk glaze ingredients and drizzle over cooled muffins. Add a few extra berries on top if desired.