flourless chocolate cake recipe

1 cup (225g) unsalted butter, cut into pieces - 1 cup (170g) bittersweet or semisweet chocolate chips or chopped chocolate - 1 1/4 cups (250g) granulated sugar - 6 large eggs, room temperature - 1 teaspoon vanilla extract - 1/4 teaspoon salt - Optional: powdered sugar or cocoa powder for dusting