For the streusel: ½ cup (110 g) packed brown sugar
¼ cup (30 g) all-purpose flour
Pinch of kosher salt
3 tablespoons unsalted butter, melted
For the cake: 2 cups (250 g) all-purpose flour
1 ½ teaspoons baking powder
1 teaspoon kosher salt
½ cup (115 g/1 stick) unsalted butter, softened
1 1/3 cups (265 g) sugar
2 large eggs plus 1 yolk
1 tablespoon vanilla extract
½ cup (120 ml) whole milk (or buttermilk if that’s your jam!)
1 tablespoon fresh lemon juice
2 tablespoons lemon zest
2 ½ cups (365 g) fresh blueberries
For the lemon icing: 1 cup (125 g) powdered sugar
2 tablespoons fresh lemon juice
Preheat the oven to 350°F (175°C). Spray a 9-inch (23 cm) cast-iron skillet with nonstick cooking spray.
To make the streusel: In a medium bowl, whisk together the brown sugar, flour, and salt. Add the melted butter and stir until a crumbly mixture forms with some large pieces still intact. Do not overmix.
To make the cake: In a small bowl, whisk together the flour, baking powder, and salt; set aside.
In the bowl of an electric mixer, beat the butter until smooth, about 1 minute. Add the sugar and continue to beat, scraping down the sides of the bowl as needed, until the mixture is light and fluffy, about 2 minutes. Add the eggs and yolk and the vanilla and beat until combined. In two additions, add the milk and the flour mixture, alternating between the two. Beat until combined. Gently fold in the lemon juice, lemon zest, and blueberries. Transfer the batter to the prepared skillet and spread into an even layer. Sprinkle the top with the streusel. Bake on the center rack until a toothpick inserted into the center comes out clean and the topping is deep golden, about 50 minutes.
To make the icing: Meanwhile, in a small bowl, combine the powdered sugar with lemon juice and stir until smooth.
Transfer the skillet to a cooling rack and allow the cake to cool in the pan for about 15 minutes, then drizzle the top with the icing. Serve warm or at room temperature.