If you opt to use unsalted butter in this ultrarich shortbread-like cake, increase the kosher salt to 1 teaspoon, otherwise the flavor will fall flat. And if you're so inclined, this is the place to splurge on high-fat, European-style butter or cultured butter. It's best to err on the side of overbaking, as the cake will have a raw, starchy flavor if underbaked. If you don’t have a springform pan, use a 9-inch-round cake pan coated with cooking spray and dusted with flour. Let the cake cool in the pan, then invert it onto a wire rack to remove.
Tip: Don't begin beating the butter and sugar until the butter is softened, then cream the mixture for a full 3 minutes. The batter needs to be very light and fluffy before the eggs are added, or it will bake up too dense.