1. Position a rack in the center of the oven and preheat to 375 F. Lightly spray a 12-cup muffin pan with the cooking spray.
2. To prepare the pans, place 2 teaspoons of the butter and 2 teaspoons of the sugar in each muffin cup. Bake until the mixture in each cup is melted and bubbly, about 10 minutes. Remove the pan from the oven and cool for a few minutes. Cut off about 1 / 4 inch from the ends of the lemons, discard and cut the rest in thin slices, about 1/8 inch. Remove the seeds. In each muffin cup, place three lemon slices on the bottom, layered over each other if need be. Dice the remaining lemons, peel and all, and reserve.
3. To make the cakes, sift together the flour, corn meal, baking powder and salt into a medium-size bowl. In a separate bowl, whisk together the sugar, oil, eggs and yogurt. Make a well in the center of the dry ingredients, add the egg mixture, and stir just until blended. Do not overbeat. Stir in the reserved diced lemons and divide evenly among the prepared muffin cups, filling them as full as possible.
4. Bake until the center of a muffin springs back when pressed gently with your finger, about 18 - 20 minutes. Immediately invert and unmold the cakes onto a wire cake rack. Cool briefly and serve. These cakes are best the day they are made or kept in an airtight container for up to three days.