You should end up with more frosting than needed. In a mixing bowl, cream butter and sugar on high speed, until light and fluffy, about 5 minutes. DIRECTIONS. In a bowl, combine flour, baking powder, and salt. Key Lime Frosting: Beat the cream cheese and butter in a bowl with electric mixer at medium speed until fluffy, about 2 minutes. Preheat oven to 350° Line muffin tins with cupcake liners or grease muffin tins and set aside. Preheat oven to 325 degrees. Next add the frosting to a piping bag with an open star tip. To thicken, put the keto key lime frosting in the refrigerator. Instructions. Sprinkle lime rind on top of your keto key lime cupcakes. Finally, swirl the frosting on each cupcake starting from the outside and working your way in. On low speed, add eggs, one at a time, mixing well after each one.