Sifting once is fine and does a great job at breaking up all of the big clumps of almond flour, cocoa powder, and powdered sugar, but sifting twice helps to make sure that no clump gets missed while giving you a velvety smooth texture of well incorporated dry ingredients to work with. I highly recommend sifting the second time just because it’s so worth it! Incidentally, I have used both the superfine almond flour from Bob's Red Mill and the Kirkland brand almond flour from Costco with great results.