Poke a hole in the top of each egg using a corkscrew opener.
After making the initial hole, peel a bit of the shell back to make a bigger hole. This will be necessary in order to get the contents of the egg out and to fill it with batter later.
Empty contents of the egg.
Make sure you set aside the contents of one egg! You will use it for the cake recipe.
Rinse the egg under the sink thoroughly.
Sink newly rinsed eggs in a salt water bath for 30 minutes. If your eggs won't sink, add more salt to the water until they do.
Set your oven to bake at 350 F.
Mix flour, baking soda, baking powder, and salt into a bowl.
In a separate bowl, mix the egg and sugar.
Add butter and lemon extract.
Add dry ingredients to wet ingredients and mix until combined.
Add sour cream and mix thoroughly.
Take eggs out of salt water bath and rinse them under the sink again using cold water. Place them hole side down on a paper towel to dry.
Place aluminum foil into cupcake tray like so. This is to keep the eggs stable and upright.
Put eggs into the cupcake tray.
Put cake batter into a piping bag that has a large tip. (I'm using a make-shift ziplock bag in case you are wondering)
Pipe eggshells 3/4 full. This part is super hard to judge but make sure you do it carefully. Too much and it will overflow an insane amount, too little and it won't fill the egg.
Once eggs are all filled, put them into the oven.
Set a timer and bake the cupcakes for 23 minutes. Oddly specific, I know.
I overfilled them! Like I said it's hard to judge. In any event, you'll have to clean them off a bit. I found that if you do this right away before it has a chance to cool, the cake comes off easier.
There we go! Not as clean as it should be, but good enough.
Finally, crack, peel, and enjoy!
Note: you can use dyed eggs! To do that simply dye the eggs like you normally would (water, vinegar, food coloring) after they have been rinsed and dried from the salt water bath.