Step 1: Preheat the oven to 350 F. Whip the egg whites with salt until holds its shape, not too stiff.
Step 2: Store in an airtight container and chill.In the same bowl, beat the butter and 50g sugar until is fluffy. Beat in the rest of sugar, lime zest and rum.
Step 3: In low speed, beat in flour and baking powder intercalating with milk just until is over the both ingredients. Turn off the beater and stir with a spatula.
Step 4: Gently, fold in the whipped egg whites, do not overmix. Divide the batter into muffin cups, leaving some space at top. Bake for 15-20 minutes or just until a toothpick is inserted comes out clean. Let cool over a rack.
Step 5: White chocolate buttercreamCream butter until is pale and fluffy. Add melted chocolate and stirred with milk. Beat until smooth. Chill to obtain consistence if the day is too hot. Spread over the cooled muffins.