Preparation

• To make the oat streusel, place the brown sugar, butter, flour, cinnamon and salt in a food processor. Pulse until the mixture resembles coarse meal. Transfer the streusel mixture to a medium bowl and gently fold in the oats. Set aside in the refrigerator. (You may transfer the streusel to an airtight container or resealable plastic bag and freeze for up to 1 month.)

• To make the cake, preheat the oven to 350 degrees. Grease an 8-inch square cake pan and set aside.

• Combine the sugar, butter and lemon zest in the bowl of a stand mixer fitted with the paddle attachment. Cream the mixture until light and fluffy, 6 to 8 minutes. Add the eggs, buttermilk and vanilla to the creamed butter mixture and mix to combine.

• In a small bowl, whisk together the flour, baking powder and salt. Add the dry ingredients to the butter mixture and mix until just incorporated, making sure to scrape down the sides of the bowl while mixing. Transfer the cake batter to the prepared cake pan, using a spatula to spread the cake batter evenly. Once the batter is evenly spread across the bottom of the cake pan, use the spatula to smear the batter up the sides of the pan, coating the entire inside of the cake pan. Spread the blueberries across the top of the cake batter, then sprinkle the oat streusel topping evenly over the blueberries to cover. (The cake can be prepared to this point and refrigerated, covered, overnight.)

• Bake the cake until it is golden brown along the edges and the center begins to bubble from the blueberries, about 45 minutes to an hour. Let cool to room temperature before slicing into wedges; the cake will sink slightly in the center from the weight of the berries. Serve at room temperature or warm slightly before serving, topping each slice with a dusting of confectioners’ sugar and a dollop of freshly whipped cream.