• Preheat the oven to 350°F. Line an 8-inch square metal pan with parchment paper or foil and spray with nonstick cooking spray.

Make the streusel:

• In a small bowl, combine all of the streusel ingredients with a fork until the texture resembles wet sand.

Brown the butter:

• In a medium, slightly deep stainless steel skillet set over medium heat, melt the butter. Swirling the pan occasionally, continue to cook the butter. It should become foamy with audible cracking and popping noises. Once the crackling stops continue to swirl the pan until the butter develops a nutty aroma and brown bits start to form at the bottom. Once the bits are amber in color, about 2 to 3 minutes after the popping stops, remove from heat and pour into a medium mixing bowl and let cool slightly. 

Make the cake:

• In a large bowl, stir together the flour, baking powder, baking soda, salt, and sugars.
• To the browned butter, add the milk, eggs, and sour cream. Make a well in the center of the dry ingredients then pour in the wet ingredients. Gently stir until just combined. Don’t overmix.
• Spoon half of the batter into the prepared pan. Sprinkle with half of the streusel. Spread the remaining batter over the streusel. Sprinkle with the remaining streusel.
• Bake for 40 minutes, or until a toothpick inserted in the center comes out clean. Cool for 15 minutes.

Make the topping:

• In a small bowl combine the sugar and milk until a thick yet pourable icing forms. Drizzle over the warm cake. Cut into squares and serve. Store leftovers, covered, at room temperature for up to 4 days.