CAKE
1 cup (2 sticks) unsalted butter, cut into pieces, plus more for baking dish
2 cups all-purpose flour, plus more for baking dish
1/4 cup unsweetened cocoa powder
2 cups sugar
1 teaspoon baking soda
1 teaspoon kosher salt
2 large eggs
1/2 cup buttermilk
1 teaspoon vanilla extract
FROSTING
1/4 cup (1/2 stick) unsalted butter
3 tablespoons whole milk
2 tablespoons unsweetened cocoa powder
1 1/2 cups powdered sugar
1 teaspoon vanilla extract
1/2 cup chopped unsalted, roasted pistachios
1 teaspoon flaky sea salt (such as Maldon)