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Preheat oven to 350 degrees. Butter and flour the baking pan. Next, whisk baking powder, salt, and flour in a medium bowl.
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Using an electric mixer on high speed, beat sugar and butter until very light and fluffy, about 5 minutes. Beat in the egg and vanilla just to combine, about 2 minutes. Reduce speed to low and slowly add the dry ingredients, then the cream. Mix just to combine and gently fold in blueberries. Scrape batter into prepared pan, smooth top, and place pan on a rimmed baking sheet. Evenly sprinkle topping over.
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Bake buckle until top is golden brown and a cake tester inserted into the center comes out clean, 80–90 minutes. Transfer pan to a wire rack and let cool before unmolding and serving.
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Enjoy! :D