FOR THE CAKE: – 2 cups (240g) all-purpose flour – 1 teaspoon baking soda – 1/2 teaspoon salt – 1 cup (226g) unsalted butter, softened – 1 & 1/4 cups (250g) granulated sugar – 3/4 cup (150g) firmly packed light brown sugar – 4 large egg – 6 ounces (170g) unsweetened chocolate, melted and slightly cooled – 1 teaspoon vanilla extract – 3/4 cup (177ml) buttermilk – 1/4 cup (56g) sour cream FOR THE FROSTING: – 1 cup (226g) unsalted butter, softened – 3 & 1/2 cups (385g) confectioners' sugar – 1/2 cup (42g) unsweetened cocoa powder – 1/4 cup (59ml) heavy cream – 1 & 1/2 teaspoons vanilla extract – 1/4 teaspoon salt – sprinkles, optional for garnishing the frosted cake