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1
Position an oven rack in the center of the oven and preheat oven to 350 degrees F. Line two 12-cup muffin tins with paper liners, 12 in one and 6 in the other.
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2
Whisk together the flour, baking powder and salt. Set aside.
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3
In the bowl of an electric mixer, cream together the butter, sugar, lemon zest and vanilla until very light and fluffy, about 3 minutes.
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4
Melt the white chocolate in a double boiler. Once melted, remove from heat and allow to cool a bit. When it's just warm to the touch, stir it into the butter mixture on low speed, just until combined-it may separate a bit, but it will come back together. With the mixer on low, alternate adding the flour mixture and the half and half to the creamed mixture in three batches until well-blended.
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5
Toss the blueberries with about a teaspoon of all-purpose flour in a small bowl, just to coat the berries with a light dusting of flour. Set aside.
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6
With a handheld mixer and a medium bowl or in a clean stand mixer bowl with the whip attachment, whip the egg whites on medium high speed until they reach firm peaks, 2-3 minutes. Stir about a third of the whites into the batter to lighten it, then gently fold in the remainder of the whites. When the egg whites are almost incorporated, fold in the blueberries.
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7
Fill the muffin cups no more than 3/4 of the way full with the batter. Bake one tin at a time until a toothpick just comes out clean, about 22-25 minutes. Invert the cupcakes onto a cooling rack, then turn them right side up and let them cool completely before frosting.
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8
For the frosting, in the bowl of an electric mixer beat together the butter and cream cheese just until smooth. Beat in the confectioners' sugar, vanilla and lemon juice. Finally, beat in the melted white chocolate, just to combine. Beat in a few drops of food coloring for a creamy yellow hue, if you wish. When the cupcakes have cooled completely, frost them generously.
