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1
To make the vanilla sugar combine the sugar and scraped seeds from the vanilla bean in a bowl and rub the vanilla seeds into the sugar until well distributed. Skip this step if you'd prefer to use vanilla extract-sub in a teaspoon in both the batter and the crumb topping.
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2
Brown the butter by cooking it in a saucepan over medium-low until it becomes foamy, stir it occasionally until the solids in the butter turn deep, golden brown and smell fragrant like toffee. Remove from the heat to cool.
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3
Preheat the oven to 350u00b0F and spray a 9-inch springform pan with cooking spray, line the bottom with a parchment circle. Set aside.
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4
To make the crumb topping stir the brown butter, sugar, flour, walnuts, salt, and cinnamon together. Set aside.
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5
To make the batter whisk together the flour, baking powder, baking soda, and salt in a mixing bowl. In another bowl whisk together the vanilla sugar, browned butter, sour cream, and eggs. Pour the sugar/butter mixture into the flour mixture and fold until just combined and there are no longer any dry streaks of flour in the batter. Spread half of the batter into the prepared pan and crumble half of the crumb topping over the batter, repeat in another layer with the remaining batter and crumb topping. Bake in the center of the preheated oven for 45-50 minutes or until a toothpick inserted into the center of the cake comes out clean. Place the cake on a wire rack to cool. Cool completely before glazing.
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6
To make the glaze, whisk together the 4 tablespoons of browned butter with the confectioners sugar and salt, plus enough milk to make it thin enough to drizzle.
