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 325 degrees. To thicken, put the keto key lime frosting in the refrigerator. Instructions. On low speed, add eggs, one at a time, mixing well after each one. DIRECTIONS. 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. Sprinkle lime rind on top of your keto key lime cupcakes. Next add the frosting to a piping bag with an open star tip. Preheat oven to 350° Line muffin tins with cupcake liners or grease muffin tins and set aside. Finally, swirl the frosting on each cupcake starting from the outside and working your way in. In a bowl, combine flour, baking powder, and salt.