Preheat oven to 375 degrees F. Line a 12-cup mini muffin tin with paper liners; set aside.
In a large bowl, combine flours, baking powder, baking soda and salt.
In a medium bowl, whisk together sour cream, milk and egg whites.
In the bowl of an electric mixer fitted with paddle attachment, beat butter, sugar and vanilla bean seeds on medium speed until light and fluffy, about 2-3 minutes. Beat in vanilla until well combined. With mixer on low speed, add dry ingredients and sour cream mixture alternately in 3 parts, beginning and ending with dry ingredients, beating just until incorporated.
Scoop the batter evenly into the muffin tray. Place into oven and bake for 15-18 minutes, or until a tester inserted in center comes out clean. Remove from oven and cool completely on a wire rack before frosting.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and confectioners’ sugar on medium speed until light and fluffy, approximately 1-2 minutes. With the mixer on low speed, gradually add heavy cream, vanilla and salt, beating just until incorporated. Increase speed to medium high and beat until frosting is light and fluffy, approximately 1-2 minutes.
Use a small offset spatula or a pastry bag fitted with a decorative tip to frost cupcakes, garnished with sparkling sugar, if desired.