Step 1

Heat the oven to 350ºF. Place 1 stick unsalted butter in an 8-inch round cake pan or 8x8-inch square baking dish. Place the pan in the oven as it preheats until the butter is melted, 5 to 8 minutes.

Step 2

Remove the pan from the oven. Swirl the pan so the butter coats the bottom and halfway up the sides; leave the melted butter in the pan while you prepare the batter. (Alternatively, melt the butter in a small saucepan over medium-low heat. Brush the inside of the pan evenly with butter, then reserve the remaining butter for the cake.)

Step 3

Place 1 cup blueberries, 1 tablespoon of the granulated sugar, and 2 teaspoons lemon juice in a medium bowl. Mash a few of the blueberries with the back of a fork, then stir to combine. Let sit for at least 15 minutes, stirring occasionally.

Step 4

Place the remaining 1/2 cup granulated sugar and the finely grated zest of 1 medium lemon in a large bowl. Rub the zest into the sugar with your fingers until fragrant. Pour in the melted butter, and add in 1/2 cup buttermilk, 1 teaspoon vanilla extract, and 1/2 teaspoon kosher salt. Whisk until combined.

Step 5

Whisk in 1 cup all-purpose flour, 1/2 teaspoon baking soda, and 1/2 teaspoon baking powder until no dry flour remains and the batter is just combined (it will be thick). Scrape into the pan and spread into an even layer. Spoon the blueberries evenly on top and drizzle any juices over the blueberries.

Step 6

Bake until the cake is golden-brown around the edges, pulling away from the sides, and a tester inserted in the center comes out clean, about 35 minutes. Let cool for at least 5 minutes. Spoon into bowls and serve with vanilla ice cream.