Cut the butter into even pieces and add to a light-colored saucepan.

Cook over medium heat, stirring constantly, until the butter turns deeply golden and smells nutty.

Immediately pour into a heatproof measuring cup, scraping in all the brown bits.

Let cool for about 10 minutes. The butter should feel warm, not hot, like bath water.

In a bowl, combine the streusel portion of brown butter with both sugars, cinnamon, salt, and vanilla.

Stir in flour until both small and large buttery clumps form.

Chill in the refrigerator while making the batter.

Preheat oven to 350°F (175°C). Line an 8×8-inch pan with parchment.

Whisk flour, baking powder, baking soda, salt, and cinnamon. Set aside.

In another bowl, whisk the cooled brown butter with both sugars.

Add eggs one at a time, then vanilla.

Whisk in sour cream and milk.

Fold in dry ingredients just until combined.

Spread half the batter into the prepared pan.

Sprinkle with half the streusel.

Spoon remaining batter gently over the top.

Finish with remaining streusel.

Bake 40–45 minutes, until a toothpick shows moist crumbs.

Cool 30 minutes.

While the cake is baking, whisk glaze ingredients until smooth. Drizzle over the warm cake.