Preheat the oven to 350°.

Set a 9-inch nonstick cake pan over moderate heat on top of the stove.

Add 4 tablespoons of butter and when it is melted, stir in the brown sugar until sugar dissolves, about 1 minute (it will be thick). Remove from the heat and using a rubber spatula spread evenly in pan.

Arrange the lemon slices on top of the melted butter-brown sugar mixture.

In a medium bowl, whisk flour, baking powder and salt.

In a large bowl, with an electric mixer, beat the remaining 8 tablespoons of butter with the granulated sugar until light and fluffy.

Beat in vanilla and egg yolks, one at a time.

Beat in lemon zest.

At low speed, beat in the dry ingredients in three batches, alternating with the milk.

In a stainless steel bowl, beat egg whites with cream of tartar at high speed until firm peaks form.

Fold one-third of beaten whites into the batter, then fold in the rest.

Scrape batter into the prepared pan and bake for about 30 minutes, or until a toothpick inserted in the center of the cake comes out clean.

Let cake cool in pan for 15 minutes on a cooling rack. Run a thin knife around the edges and invert onto a plate.

Serve warm or at room temperature with freshly whipped cream, vanilla ice cream or crème fraîche mixed with zest from one lemon.