1 cup all purpose flour
1 1/2 teaspoons baking powder
1/4 teaspoon salt
3/4 cup unsalted butter, softened, divided use
1/3 cup + 1/2 cup granulated sugar, divided use
1 1/2 teaspoons finely grated orange zest
2 large eggs
1/3 cup milk
1/3 cup fresh orange juice
1 teaspoon vanilla extract
Heat oven to 350 degrees F. Coat a (24 cup) mini muffin pan with cooking spray.
In a medium mixing bowl, sift together flour, baking powder and salt. In a large mixing bowl, using a mixer on medium speed, beat together 1/4 cup butter, 1/3 cup sugar and orange zest until creamy - about 1 minute. Crack open the eggs and place whites in a large heat-proof mixing bowl. Add yolks to mixing bowl with batter in it, along with milk and orange juice; beat 1 minute or until combined. Reduce mixer speed to low and beat in flour mixture until just combined.
Place batter into each muffin cup. Bake 10-12 minutes or until cake springs back when lightly touched. Cool babycakes 5 minutes before transferring to a wire rack to cool completely.
To make the buttercream, Place bowl with egg whites over a pot of simmering water; whisk in remaining 1/2 cup sugar. Cook, whisking frequently, until the mixture reaches 160° F and the sugar has dissolved. It will look thick and marshmallow-y.
Beat mixture using a mixer on medium-high speed until stiff peaks form and the mixture has cooled to room temperature, about 8 minutes.
Reduce speed to medium and add remaining 1/2 cup butter, a tablespoons at a time, until well incorporated and fluffy. If the frosting looks curdled, continue to beat on medium-high speed until thick and smooth again, about 3-5 minutes more. Beat in vanilla until combined.
Pipe buttercream on top of cupcakes and eat!