Preheat oven to 350°. Lightly grease an 8-or 9-inch cast-iron skillet. Met 4 tablespoons butter in skillet over medium heat. Stir in brown sugar until sugar is melted then remove from heat. Slice 2 of the lemons into 1/4-inch thick slices, remove seeds. Arrange the slices on top of the melted butter and sugar.

In a medium bowl whisk together flour, baking powder, baking soda, and salt. In a large bowl or bowl of a stand mixer add sugar and zest of 2 remaining lemons and rub them together with your fingers. Juice remaining 2 lemons and in a small bowl combine 1/4 cup of the lemon juice with poppy seeds.

Add 1 stick butter to the bowl with sugar and beat for 5-6 minutes on medium speed until fluffy and color lightens. Scrape down the sides of the bowl then mix in eggs and yolks, one at a time. Gradually mix in dry ingredient mixture. Finally, mix in sourdough starter, then lemon juice and poppy seeds. Scrape down the sides of the bowl again and mix just until everything is evenly incorporated.

Spoon batter into the skillet over lemon slices. Bake for 35-45 minutes until a toothpick inserted in the center comes out clean. Let sit for 10 minutes then carefully run a butter knife along the edge of the pan. Invert skillet onto a plate, give the bottom a few taps then remove pan. If any caramel bits stuck to the pan you can scrap them off and place on top of cake. Serve warm or room temperature, cut with a serrated knife.