In a large skillet, heat the oil to at least 290 degrees. Carefully drop four of the biscuits into the oil and cook on each side for about 1.5-2 minutes until lightly browned on each side. Place on a cookie sheet lined with paper towels and let cool off. Put a couple of cups of powdered sugar into a large plastic Ziploc bag. Place the doughnuts, one at a time in the bag and shake until they are covered with powdered sugar. Set aside on a large cooling rack.
To Make the Buttercream
Beat the first four ingredients at medium speed with an electric mixer until creamy. Gradually add powdered sugar alternately with key lime juice, beating at low speed until blended and smooth after each addition. Beat in green food coloring (if using).
With a pastry tip inserted into a pastry bag or large Ziploc bag, fill with buttercream and insert into the side of the doughnut. Squeeze and fill with buttercream. The doughnut will get noticeably "puffier" and expand a bit from the buttercream. (One of mine even burst a little on one side because I got slightly carried away, but that was ok..it still tasted good.) After they have all been filled, dredge one last time in powdered sugar. Eat immediately or store in an air-tight container.
Notes
Makes 8-16 doughnuts.
*How many doughnuts you want will depend on how full you fill them. I only made 8 and I had some frosting leftover but if I had done 12 or even 16, I would have just filled with a little less buttercream in each one and it would have filled all of them.
Recipe by DessertedPlanet.com at https://dessertedplanet.com/key-lime-buttercream-filled-doughnuts/