Once puree is cool, place the butter in the bowl of a stand mixer fitted with the paddle attachment. Beat butter until very smooth, then add 2 1/2 cups powdered sugar, 1 tablespoon milk, orange juice, orange zest, and salt and mix to combine. Beat in the puree until combined, then add food coloring, if using. Scrape down the sides and bottom of the bowl, then turn mixer to high and beat 3 to 4 minutes, until very fluffy. Add additional powdered sugar if frosting is too thin and additional milk if frosting is too thick. Frost and decorate cupcakes as desired.