In a large bowl, mix sifted flour, baking powder, baking soda, salt, and cinnamon. Set aside.
Make sure all ingredients are room temperature
In a stand or hand mixer with the whisk attachment, whisk butter on high (about a minute) until creamy.
Add sugar and mix until combined, about 1-2 minutes.
Add egg whites and continue to whip until mixed. Mixture will begin to get fluffy.
Add vanilla and sour cream and mix until combined.
Reduce speed to low, and mix in dry ingredients until just combined. Do not overmix.
Slowly pour in horchata and mix until just combined.
Fill cupcake liners ¾ full, and bake for 15-18 minutes, or until toothpick comes out clean when inserted.
Let cool completely before icing.
Buttercream
In a stand mixer with the whisk attachment, whisk together butter and cream cheese. For this, I used cold cream cheese so it would not be too soft. Mix until combined and creamy.
Mix in powdered sugar.
Add cinnamon, vanilla, and horchata. Mix until smooth and creamy. Pipe onto cupcakes and enjoy!