Make sure there is a rack in the middle of the oven & preheat oven to 350 degrees F.
Trim ends from lemons & cut into slices 1/8" to 1/4" thick, discarding any seeds.
In an ovenproof, nonstick 10" skillet over medium heat, melt unsalted butter with the brown sugar.
Add lemon slices & increase heat to high. Boil & stir for 1 minute.
Remove skillet from heat & arrainge slices in an attractive pattern in the bottom of the skillet.
In a medium bowl, whisk together flour, baking powder & salt.
In a large bowl, using an electric mixer on medium speed, beat the remaining 1/2 cup of butter until light & fluffy.
Add granulated sugar & zest, again beating until light & fluffy (& scraping down the side of the bowl).
Add egg yolks & vanilla & beat just until blended, then reduce speed to low & add flour mixture alternately with the milk, scraping down the side of the bowl after each addition. Beat until just blended.
In a large clean bowl, beat egg whites with clean beaters on medium speed until just foamy, then increase speed to medium-high & add cream of tartar, beating just until egg whites form stiff peaks.
Add 1/4 of the whites to the batter & fold in using a whisk or rubber spatula ~ Continue to gently fold in the whites, 1/4 at a time, being careful not to overmix.
Pour batter over lemon slices & gently smooth the top, then bake 45-50 minutes or until top is golden brown & a wooden pick inserted in the center comes out clean.
Let cool in the pan on wire rack for 10 minutes.
Loosen edge of cake with a rubber spatula all around, then invert it onto a serving platter, leaving the pan over the cake for 5 minutes.