In a small, light-colored saucepan over medium heat, melt the butter. Continue cooking, swirling the pan occasionally, until the butter foams, then turns a deep golden amber color and smells nutty (about 5-7 minutes). Watch carefully to prevent burning.

Immediately pour the brown butter into a medium mixing bowl, scraping all the browned bits from the bottom of the pan.

Add the flour, dark brown sugar, cinnamon, and cardamom to the bowl. Use a fork or your fingers to mix until the mixture resembles coarse, wet sand and starts to clump together. If using nuts, fold them in now. Set the streusel aside to cool slightly while you make the cake batter.

Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan with butter or non-stick spray. For easy removal, you can also line the bottom with parchment paper.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter and granulated sugar on medium speed until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, beating well after each addition. Scrape down the sides of the bowl as needed. Beat in the vanilla extract.

In a small separate bowl or a large measuring cup, whisk together the sour cream and whole milk until smooth.

With the mixer on low speed, add the dry ingredients to the butter mixture in three additions, alternating with the sour cream mixture in two additions. Begin and end with the dry ingredients. Mix on low until just combined. Do not overmix; the batter should be smooth and thick.

Spread half of the cake batter evenly into the prepared pan. Use an offset spatula to smooth the top.

Sprinkle half of the brown butter streusel evenly over the batter.

Carefully dollop the remaining batter over the streusel layer and gently spread it to cover the filling as best you can (it's okay if some streusel peeks through).

Sprinkle the remaining streusel over the top of the batter.

Bake in the preheated oven for 45-50 minutes, or until a wooden toothpick inserted in the center comes out clean with a few moist crumbs.

Place the pan on a wire rack and allow the cake to cool completely in the pan.

In a small bowl, whisk together the powdered sugar, 2 tablespoons of milk, and vanilla extract until smooth. Add the remaining tablespoon of milk if you want a thinner consistency.

Drizzle the glaze over the cooled cake in a zig-zag pattern.

Slice the cake and serve. This cake is best enjoyed the day it's made but stores well, covered, at room temperature for up to 3 days.