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Sift together the flour, baking powder, baking soda, and salt into a medium bowl, set aside.
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In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar together at medium speed until light and fluffy, 3 to 4 minutes. Beat in the eggs, one at a time, beating well after each addition, then beat in vanilla and sour cream. Scrape down the sides of the bowl. Add the dry ingredients and beat on low speed until just combined.
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Scrape the batter into the prepared pan, smoothing the top with a spatula. Using your hands, sprinkle the topping evenly over the batter, making sure some of the larger clumps remain as you go and covering the top completely.
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Bake for 30 to 35 minutes, until a wooden skewer inserted into the center of the cake comes out clean and the crumb topping is firm. Transfer to a wire rack and let cool for 10 to 15 minutes. Sprinkle with confectioners' sugar. Cut into squares and serve. The cake will keep, tightly covered at room temperature for up to 2 days.