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1
Preheat oven to 400F.
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2
Line standard muffin tins with paper liners.
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3
Place 3 whole unpeeled bananas on a baking sheet and roast 15 minutes (the peels will darken).
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4
Meanwhile, sift together cake flour, baking soda, baking powder, and salt.
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5
Remove bananas from oven and let cool before peeling.
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6
Reduce oven temperature to 350F.
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7
With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy.
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8
Add egg yolks, one at a time, beating until each is incorporated, scraping down sides of bowl as needed.
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9
Add roasted bananas, and beat to combine.
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10
Add flour mixture in three batches, alternating with two additions of sour cream, and beating until just combined after each.
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11
Beat in vanilla.
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12
In another mixing bowl, with electric mixer on medium speed, whisk egg whites to soft peaks; fold one-third whites into batter to lighten.
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13
Gently fold in remaining whites in two batches.
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14
Divide batter evenly among lined cups, filling each three-quarters full.
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15
Bake, rotating tins halfway through, until a cake tester inserted in centers comes out clean, about 20 minutes.
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16
Transfer tins to wire racks to cool completely.
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17
Cupcakes can be stored up to 3 days at room temperature, or frozen up to 2 months, in airtight containers.
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18
To finish, fill a pastry bag fitted with a medium French-star tip (Ateco #863 or Wilton #363) with frosting.
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19
Pipe seven scallops around perimeter of cupcake, and pipe a large starburst in the center.
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20
Just before serving, thinly slice remaining bananas, and place 2 pieces on top of each cupcake.
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21
With an electric mixer on medium speed, beat all ingredients until smooth.
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22
Use immediately, or refrigerate up to 5 days in an airtight container; before using, bring to room temperature and beat until smooth.
