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Preheat the oven to 350 F.
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Butter a 9 inch springform pan thoroughly. Set aside.
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Whisk together the flour, baking powder, baking soda, cream of tartar, salt and nutmeg. Set aside.
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Beat the butter on moderate speed until creamy. Add the orange and lemon zest and beat an additional 30 seconds. Scrape the sides and bottom of the bowl.
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Add the sugar, gradually, while beating the butter on moderate speed. When all of the sugar is added, beat an additional minute, until light and fluffy.
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Scrape the sides and bottom of the bowl again.
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Add the eggs, one at a time, scraping the bowl as needed. Beat for 30 seconds after adding each egg, incorporating the eggs into the batter. Beat in the vanilla if using.
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Remove 1 tablespoon of the flour mixture to a medium sized bowl. Set aside.
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On the lowest speed, add the flour mixture and sour cream, alternating, in 4 and 3 additions, respectively. Begin and end with the flour mixture.
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Toss the frozen blueberries with the tablespoon of flour. Then gently fold the blueberries into the cake batter by hand.
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Scrape the batter into the prepared springform pan. Sprinkle the unbaked crisp topping over the cake evenly.
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Bake for 50-60 minutes, until the topping is brown and crisped, and a cake tester inserted into the center of the cake comes out with only a few crumbs or blueberry juice on it.
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Let the cake cool in the pan for 10 minutes. Then remove the outer wall of the springform pan and let cool completely.