Cream butter and add sugar beating until well blended. Add flavoring and egg yolks to butter mixture and beat well.
Add sour cream and mix well. Fold in dry ingredients. When incorporated remove from mixer and set aside.
Beat egg whites in a clean bowl until fluffy. Fold into the batter until well mixed. Do not over mix.
Heat oven to 350 degrees and use an ice cream scoop to fill paper cups 2/3 full. Bake for 18 - 20 minutes until light golden.
Frosting
Beat softened butter with sifted powdered sugar, 1 cup at a time. Add extracts and salt. Beat for 3 - 5 minutes. Add more extract to taste.
Fill with Key Lime Curd and frost when cupcakes have cooled.
Optional: Dust with graham cracker crumbs
Using a citrus zester, zest key limes on top of graham crackers
Notes
These treats would also be perfect for your next tea party. I like to use mini cupcakes for my tea parties and I put 3 small desserts on a plate. I usually place a cupcake, maybe a chocolate-dipped strawberry and then either a cookie or a mini cheesecake.