Preheat oven to 350˚F.
Buttercream: Place butter into a bowl of a stand mixer or into a large mixing bowl and cream. With the motor running, add 2 cups of powdered sugar, ½ cup at a time, to the butter until fully incorporated and smooth. Stir in the grapefruit juice until fully incorporated. Add the remaining powdered sugar, ½ cup at a time until you have a good buttercream consistency. Set aside until ready to use. (if using at a later time, store in an airtight container and re-whip before using for best consistency)
Cupcakes: Place flour, baking powder and salt in a mixing bowl and whisk together. Set aside.
Place butter and sugar in the bowl of a stand mixer or in a mixing bowl and cream together. Add eggs, one at a time, scraping down the sides of the bowl after each addition. Fold in the vanilla and almond extract.
Add the flour mixture to the butter mixture and beat together until fully combined. Stir in almond milk until a loose batter has formed.
Line a muffin tin with cupcake liners and fill each liner ⅔ of the way with the batter.
Bake cupcakes for 22 to 25 minutes or until a toothpick comes out clean when inserted into the center of the cupcake.
Allow cupcakes to cool completely before frosting.
Finish the cupcakes with fresh grapefruit zest and serve.