Preheat oven to 350oF. Line muffin pan with paper cups.
In a bowl, cream margarine and sugar with an electric mixer until light and fluffy. Add eggs one at a time then add vanilla, calamansi juice and zest.Mix well.
In a separate bowl, combine flour with baking powder. On low speed, gradually add flour mixture to margarine mixture until well blended.
Scoop batter onto prepared muffin pan and bake for 20 minutes. Let cool.
In a bowl, beat cream cheese and margarine until fluffy. At low speed, add powdered sugar and milk. Continue beating until well combined. Using a rosette tip, top each cupcake with prepared frosting. Decorate with colored sprinkles or calamansi slices if desired.