Special equipment: one 12-cup standard muffin tin or one 24-cup mini muffin tin cupcake liners
Make the cupcakes: Position an oven rack in the center of the oven, and preheat to 350 degrees F. Line one 12-cup standard muffin tin or one 24-cup mini muffin tin with cupcake liners, and spray with cooking spray.
Whisk together the flour, baking powder and salt in a medium bowl. Whisk the eggs and sugar in a large bowl until light and foamy, about 1 minute; while still whisking, gradually add the butter, then the vanilla and orange zest. Add half the dry ingredients to the wet ingredients, and stir with a spoon to combine. Add the milk, then the remaining dry ingredients, and stir to combine, but don't overmix.
Divide the batter evenly among the cups in the prepared tin. Bake until a toothpick inserted in the center of the cakes comes out clean, 18 to 20 minutes (10 to 12 minutes for minis), rotating the tin halfway through. Let the cupcakes cool in the tin on a cooling rack for 10 minutes, then remove them from the tin and let them cool completely on the rack.
Meanwhile, make the frosting: Process the avocado flesh and the butter in a food processor until completely smooth. With the processor still running, slowly pour in the cream. Add the confectioners' sugar, and pulse until light green and completely combined, about 1 minute. Cover and refrigerate until ready to use. Spread the frosting on the cooled cupcakes.
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