Preheat the oven to 350 degrees.
Separate egg whites and yolks from 4 of the eggs into 2 bowls. Combine 2 cups of the sugar and the butter in the bowl of an electric stand mixer and mix until blended. Add the remaining 2 whole eggs and all of the egg yolks to the mixer one by one, making sure each one is incorporated before adding another.
Combine the flour, baking powder, and salt in a bowl and the buttermilk and lemon juice in another. Slowly add the wet and dry ingredients to the mixer, alternating each time. Butter a cupcake pan or line with cupcake wrappers. When the ingredients are mixed, place a dollop of the mix into each cup. Place the cupcakes in the oven for 15 minutes.
Meanwhile, slice the lemons and remove the rind so you are left with circles of lemon. Place on a baking sheet and sprinkle sugar on top. Place in a toaster oven on broil for 5 minutes (if you do not have a toaster oven, you can do this step prior to baking the cupcakes and then reduce the heat back to 350 degrees before putting in the cupcakes).
Clean out the bowl of the stand mixer. Add the egg whites to the bowl and mix at high speed. Slowly add the remaining sugar as the meringue develops. The meringue is done when there are stiff peaks.
After the cupcakes have been in the oven for 15 minutes (they should be forming but not done), remove from the oven and add a lemon slice and then a dollop of meringue onto each one. Place back in the oven for 5-10 minutes until the cupcakes are done and the meringue is browned. Serve warm.