Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners. This ensures easy release and keeps the streusel intact.

Make the streusel: In a medium bowl, whisk together the brown sugar, flour, cinnamon, and salt. Add the cold butter cubes. Use a fork or your fingertips to cut the butter into the dry mixture until it resembles coarse, pea-sized crumbs. Do not overwork it — you want distinct clumps. Set aside.

Make the muffin batter: In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In another large bowl, whisk the melted butter and granulated sugar until combined — it will look like wet sand. Whisk in the egg and vanilla until smooth. The mixture should be pale and slightly thickened.

Add half the dry ingredients to the wet mixture, then half the buttermilk. Stir gently with a rubber spatula just a few times. Add the remaining dry ingredients and buttermilk, and fold until just combined. A few streaks of flour are fine — overmixing makes tough muffins. The batter will be thick and slightly lumpy.

Divide the batter evenly among the prepared muffin cups, filling each about 2/3 full. Use a small cookie scoop or two spoons for neat portions.

Generously sprinkle the streusel over each muffin, pressing it gently into the batter so it sticks. Use all the streusel — pile it high; it will sink slightly as the muffins rise.

Bake for 16-18 minutes, until the muffins are golden brown on top and a toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs. The tops should spring back when lightly touched. Cool in the pan for 5 minutes, then transfer to a wire rack.

For the glaze (optional): In a small bowl, whisk together the powdered sugar and 1 tablespoon of milk until smooth. Add more milk a teaspoon at a time if needed for a drizzling consistency. Drizzle over warm muffins.

Serve warm or at room temperature. Store leftovers in an airtight container at room temperature for up to 3 days.