Heat the oven to 350°F. Place 1 stick unsalted butter in an 8x8-inch square baking dish. Place in the oven while it is heating until the butter is melted, 4 to 5 minutes. (Alternatively, melt the butter in the microwave and pour into the baking dish.)

Meanwhile, whisk 1 cup all-purpose flour, ¾ cup granulated sugar, 1 tablespoon lemon zest, and 1 ¼ teaspoons baking powder together in a large bowl.

Slowly whisk in 1 cup room-temperature buttermilk, 1 teaspoon vanilla extract, and ¼ teaspoon kosher salt until combined (some small lumps are OK).

Transfer to the baking dish but do not stir. Dollop 1 cup lemon curd evenly over the batter.

Bake until golden-brown and bubbling around the edges, 40 to 45 minutes.

Let cool for at least 20 minutes before serving so the lemon curd can set. Serve warm or at room temperature. Dust with powdered sugar or serve with vanilla ice cream, whipped cream, or blueberries if desired.