Lightly spray with baking oil and flour two 8-by-2-inch cake pans. Make sure to tap out the excess flour. In a bowl, whisk together flour, baking powder, baking soda, salt, and lemon zest.
In a large bowl mixing bowl and using an electric mixer, or use your KitchenAid mixer like me, cream together butter and 1 1/2 cups sugar until light and fluffy. Reduce mixer speed to low and add in eggs and yolks, one at a time. Mix in lemon juice. Alternate adding in flour mixture and buttermilk beginning and ending with flour. Mix until well combined.
Pour ½ of the batter into one pan and add the remaining to the second pan. Place in oven and bake for 30-35 minutes or until cake starts to pull away from the sides of the pan.
Once cakes are completely cooked remove from the oven and cool. Allow the pans to cool for 10 minutes on a cake rack, then remove cakes from the pans and place back on cake rack, allowing them to completely cool.
While the cakes are cooling, make lime buttercream. In a large bowl, cream together the confectioners' sugar and butter until smooth. Gradually mix in the milk, lime zest and juice with an electric mixer until smooth and stiff, about 5 minutes. Color with food coloring if desired.
*2 Layer Cake -* Evenly spread the filling on top of the bottom layer, then add the second cake on top.
You can also garnish the top of the cake with fresh or candied lemon slices, if desired.