To make the cupcakes, preheat an oven to 350 degrees F. Line 12 muffin cups with paper liners. In a large bowl, sift together flour, baking powder, and salt. In the bowl of a stand mixer fitted with the paddle attachment, beat together the sugar and butter until light and fluffy, about 2 to 3 minutes. Add egg yolks, one at a time, mixing until well blended. Add vanilla, lemon juice and lemon zest and beat until combined. Slowly add the flour mixture in 3 additions, alternating with the coconut milk, beginning and ending with the flour mixture. Scrape down the sides of the bowl as needed and beat until combined.

In another clean bowl, beat the egg whites until soft peaks form. (You can use a stand mixer or hand mixer.) Stir one-fourth of the egg whites into the cake batter to lighten, then fold in the remaining egg whites until combined. (Be careful not to over mix. You want some egg whites to still be visible in the batter.) Divide the batter evenly among the lined muffin cups, filling them about 3/4 full. Bake until a toothpick inserted into the center of the cupcake comes out clean, about 20 minutes. Let cool for 5 minutes in pan, then transfer to wire rack to cool completely.

To make the frosting, in the bowl of a stand mixer fitted with the paddle attachment, beat butter until light and fluffy, about 1 minute. Slowly add 2 1/2 cups confectioners' sugar and beat until fluffy, about 2 to 3 minutes. Add lemon zest, lemon juice and remaining sugar. Beat for an additional one to two minutes.

Pipe frosting onto each cupcake and sprinkle with coconut. Garnish with sliced lemons, as desired.