Preheat oven to 325 degrees.

Line cupcake pans with 18 paper cupcake liners. (If you are using standard 12-cup pans, you will have 6 unlined cups.) Set aside.

Sift together the flour, baking powder, baking soda, and salt in a bowl.

In the bowl of a stand mixer, beat the pureed avocado, with the sugar until completely combined. Beat in vanilla. Beat in eggs one at a time, mixing well after each addition. With the mixer on low speed, beat in all of the buttermilk until just combined.

Add the flour mixture in 3 additions, mixing each addition until just combined. Once all flour, is incorporated, increase mixing speed to medium and and beat for 20 seconds longer. Be careful to not over mix.

Divide batter among the prepared cupcake liners.

Bake until golden on top and a wooden pick inserted in center comes out clean, about 18-20 minutes.

Cool cupcakes in pans on racks for 10 minutes then remove from pans and cool completely before frosting.

While the cupcakes are cooling, prepare the buttercream.

In the bowl of a stand mixer, with speed set on medium high, cream butter with avocado, cream cheese and lime zest for 3 minutes, stopping several times to scrape the sides of the bowl. (Beating for this length of time is important for ensuring a very creamy icing).

Reduce the speed to low and gradually add confectioner's sugar. Once all the confectioner's sugar is incorporated add lime juice and mix well. Add vanilla and then milk, a few drops at a time until frosting is soft and fluffy. Increase speed to medium high and beat for another 5 minutes. If frosting is too thin add a little more confectioner's sugar, if too thick add a little more milk. Add food coloring, if desired.

Frost cupcakes with a knife or a decorating bag. Any extra frosting can be stored in a refrigerator for up to a week. Store in an airtight container. Bring to room temperature before using.