Cupcake:

2 sticks unsalted butter, at room temperature
2 cups, plus 2 tbsp granulated sugar
4 large eggs, plus 1 white at room temperature
3 cups GF flour (3/4 cup white rice flour, 1 cup sweet rice flour, 3/4 cup potato starch, 1/2 cup GF oat flour)
1/2 teaspoon baking powder
1 teaspoon baking soda
1 teaspoon kosher salt
2 tbsp lemon juice
3/4 cup sour cream, room temperature
1/4 cup orange juice
3 tbsp orange zest
2 tbsp lemon zest
1 teaspoon pure vanilla extract

Orange Curd:

2 oranges
1 lemon 
1 cup, plus 1 tbsp sugar
1 stick unsalted butter, room temperature
4 large eggs
1/4 cup, plus 1 tbsp orange juice
3 tbsp lemon juice
Pinch of kosher salt

Buttercream Frosting:

4 egg whites
1 cup granulated sugar
Pinch of salt
3 sticks unsalted butter
1/2 tbsp vanilla bean paste
Juice from 1 whole orange and 1/2 lemon