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1
Make the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin pan or two 24-cup mini muffin pans with paper liners; lightly spray the liners with cooking spray.
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2
Soak the prunes in a bowl with 3/4 cup hot water until softened, about 10 minutes.
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3
Pour into a blender and puree until smooth.
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4
Whisk the all-purpose flour, whole-wheat flour, cocoa powder, baking powder, baking soda, salt and cinnamon in a large bowl.
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5
In another bowl, whisk the prune puree, brown sugar, eggs, yogurt, vegetable oil and vanilla.
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6
Fold the wet ingredients into the dry ingredients until just combined (it's fine if there are a few lumps).
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7
Divide the batter among the muffin cups, filling each three-quarters of the way.
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8
Bake until a toothpick comes out clean, 18 to 20 minutes for standard-size cupcakes, or 12 to 14 minutes for minis.
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9
Transfer to a rack and let cool in the pan, 10 minutes, then remove from the pan to cool completely.
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10
Make the frosting: Whisk the egg whites, granulated sugar, cream of tartar and salt in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water) until the sugar dissolves, 1 to 2 minutes.
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11
Remove the bowl from the pan.
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12
Transfer the egg white mixture to a stand mixer fitted with the whisk attachment; beat on medium-high speed until stiff peaks form, 3 to 5 minutes.
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13
Sift in the cocoa powder and add the vanilla and orange zest; beat to combine.
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14
Spread the frosting on the cupcakes.
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15
Photograph by Kang Kim
