Preheat oven to 375 degrees F. Line two 6 inch cake pans with parchment paper. (You can also use 8 or 9 inch pans).

Put your pistachios in a food processor and pulse until you get a fine crumb. Put the crumbs into a large bowl and whisk in flour, baking powder and baking soda. Reserve some of the crumbs if you wish to garnish with them later.

With a stand mixer fitted with the paddle attachment beat the butter and sugar until light and fluffy. Beat in egg whites until incorporated. Scrape down the sides of the bowl. Beat in sour cream, vanilla and almond. Slowly mix in dry ingredients until incorporated. Turn mixer to low and slowly pour in milk.

Divide batter evenly between cake pans. Bake for 25-30 minutes or until a toothpick inserted comes out clean. Baking time will depend on how much you fill you pans. More batter = longer bake time. Just keep an eye on them.

Let cool in pans for 15 minutes before moving to a wire rack to cool completely.

Beat butter in a stand mixer until smooth. Slowly add in powdered sugar. Once combined add in rosewater extract. Add in milk a tablespoon at a time until desired consistency is reached.

Color your frosting with gel food coloring if desired.

If you made larger cakes and plan to decorate with buttercream flowers, you may need to double this recipe.

Level you cakes so they are flat. You can either then cut your layers in half or leave them whole for larger layers.

Alternate between cake layer and then frosting until all layers have been used. Then frost the cake completely. Depending on the amount of layers, you may have to double the frosting recipe. Garnish with remaining pistachio crumbs.