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Preheat the oven to 350°F. Lightly grease (or line with parchment) a 9” square pan.
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To make the streusel: In a medium bowl, stir together the sugars, salt, flour, and spices.
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Work in the cold butter until the mixture is evenly crumbly, then stir in the chopped walnuts. Set aside.
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To make the cake: In a large bowl or the bowl of a stand mixer, combine the flour, sugar, salt, and baking powder.
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Cut the butter into small pieces and add to the bowl of dry ingredients. Beat at low speed until the mixture is sandy looking.
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In a separate bowl, whisk together the egg, milk, extracts, and lemon zest. Add the liquid ingredients to the dry ingredients and mix until combined. Note: the batter will be very thick.
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Using a spatula, carefully fold the blueberries into the batter.
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Spoon the batter into the prepared pan, gently spreading it evenly across the bottom.
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Sprinkle the streusel topping over the batter, shaking the pan to distribute it evenly.
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Bake the cake for 45 to 50 minutes, until it's brown on top and a toothpick inserted into the center comes out clean.
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Remove the cake from the oven and allow it to cool to room temperature, right in the pan, before serving.
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Storage information: Store any leftover cake at room temperature, covered, for up to a week. Freeze for longer storage.