Preheat oven to 350°F. Line a 9-inch springform pan with parchment paper and grease with nonstick spray.
In a medium bowl, combine the all-purpose flour, baking powder, baking soda and salt.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and brown sugar together until light and creamy, about 2-3 minutes. Add the eggs one at a time, mixing after each, followed by the vanilla.
Add the dry ingredients in 2 additions, alternating with the buttermilk, scraping down the bowl as needed. Stir in the mini chocolate chips. Spread into the prepared cake pan.
To make the streusel topping, cook the butter over low heat until browned, about 5 minutes. Let cool slightly.
Combine the brown butter, brown sugar and flour with a fork in a small bowl. Sprinkle over the cake batter. Bake until a toothpick inserted into the center comes out clean, 35-40 minutes.
Let cool in the pan 15 minutes, then run a knife around the edges and remove springform pan sides. Let cool completely.
To make the icing, whisk together the powdered sugar and milk. Drizzle over cooled cake and let sit 10 minutes before serving.