For the cupcakes:
- ¾ cup plus 2 tbsp. all-purpose flour, divided
- ¾ cup cake flour
- 1½ tsp. baking powder
- ¼ tsp. salt
- 8 tbsp. unsalted butter, at room temperature
- ¾ cup plus 2 tablespoons sugar
- Zest of 1 lemon
- 2 large eggs, at room temperature
- 1 tsp. vanilla extract
- 2 tbsp. lemon juice
- ½ cup plus 2 tbsp. milk, at room temperature
- 1 cup fresh blueberries
For the frosting:
- 3 large egg whites
- 1/2 cup plus 2 tbsp. sugar
- Pinch of salt
- 1 sticks unsalted butter, at room temperature (more if too thin)
- zest of 1 lemon
- 1 tbsp fresh lemon juice
- 1 tsp. vanilla extract
Preheat the oven to 350˚ F. Line cupcake pans with 16 paper liners. In a medium bowl, combine the ¾ cup all-purpose flour, the cake flour, baking powder and salt; whisk together and set aside. In the bowl of an electric mixer, combine the butter, sugar and lemon zest. Beat together on medium-high speed until light and fluffy, about 2-3 minutes. Beat in the eggs one at a time, blending well after each addition. Beat in the vanilla and lemon juice. With the mixer on low speed, mix in half of the dry ingredients just until incorporated. Blend in the milk. Mix in the remaining dry ingredients, beating just until incorporated.
In a small bowl, toss the blueberries with the remaining 2 tablespoons of all-purpose flour. Using a spatula gently fold the berries into the cake batter. Divide the batter evenly between the prepared liners, filling each about ¾ full. Bake 20-22 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pans 5 minutes, then transfer to a wire rack to cool completely.
To make the frosting, combine the egg whites, sugar and salt in a heatproof bowl set over a pot of simmering water. Heat, whisking frequently, until the mixture reaches 140° F and the sugar has dissolved.
Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed until stiff peaks form and the mixture has cooled to room temperature, about 8 minutes (12 if using a hand mixer).
Reduce the speed to medium and add the butter, 2 tablespoons at a time, adding more once each addition has been incorporated. If the frosting looks soupy or curdled, continue to beat on medium-high speed until thick and smooth again, about 3-5 minutes more. Mix in the lemon zest and juice and vanilla.
Keep buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes.
From Annies Eats