Make the Brown Butter:
In a medium skillet, melt 1 ½ cups unsalted butter over medium heat. Cook until the butter turns a deep amber color and becomes fragrant (about 12-15 minutes). Pour into a shallow dish and cool to room temperature.
Prepare the Streusel:
In a small bowl, combine ¾ cup crushed Biscoff cookies and 1 tablespoon flour. Add 4 tablespoons cold butter and use a pastry cutter to incorporate until crumbs form. Bake at 350°F (175°C) on a sheet pan for 10 minutes until golden brown and fragrant. Let cool and break into small pieces.
Make the Cake Batter:
In a stand mixer, cream together brown butter and 2 ¾ cups sugar for 5-6 minutes. Add 6 eggs, one at a time, and 1 tablespoon vanilla extract. Mix until light and fluffy. Sift together 2 ¾ cups cake flour, 1 ¼ teaspoons kosher salt, ¼ teaspoon baking soda, and 1 teaspoon ground cinnamon. Add this dry mixture in two parts, mixing gently.
Add Sour Cream and Cookie Butter:
Add ¾ cup sour cream and ½ cup Biscoff cookie butter to the batter. Mix until just combined.
Assemble the Cake:
Grease a 12-cup Bundt pan. Pour half the batter into the pan, add 1/3 of the streusel and ½ of the brown sugar, and swirl with a toothpick. Top with the remaining batter, and swirl in the remaining streusel and brown sugar.
Bake:
Bake at 325°F (163°C) for 1 hour and 5-10 minutes, or until a toothpick inserted comes out clean. Cool in the pan for 10 minutes, then invert onto a cooling rack and cool completely.
Make the Glaze:
Microwave ¼ cup Biscoff cookie butter for 15-20 seconds to soften. Stir in 1 teaspoon vanilla extract, 1 cup powdered sugar, 3 tablespoons milk, and a pinch of salt. Whisk until smooth and pourable.
Glaze and Serve:
Pour the glaze over the cooled cake and garnish with the remaining streusel. Let it set before serving.