In the bowl of a large mixer, whisk together the eggs and egg white powder (or meringue powder) until frothy. Add the granulated sugar and whisk on high until stiff peaks form, adding your lemon juice at the last minute.
Using a food processor, pulse your almond flour, powdered sugar, and lemon zest until smooth using 2-3 second intervals. Gently fold your dry ingredients into your meringue until just combined.
Spoon your macaron batter into a piping bag and pipe onto a parchment paper lined baking sheet. For the sake of uniformity, it’s a good idea to use a template to make sure they’re all about 1.5″ across (I made my own using a 1.5″ round cookie cutter to trace circles onto a piece of paper). Holding your baking sheet firmly, slam it onto the counter 2-3 times to work out the air bubbles. Allow the macarons to dry slightly before baking by leaving them on the counter for 30-60 minutes. Bake them at 325F/160C for 9-11 minutes, or until they easily pull away from the parchment paper.
To make the cream filling, simply whip the heavy whipping cream until stiff peaks form. You can sweeten the macarons by adding some sugar or vanilla extract to your filling, but I kept mine neutral so the tartness in the shells could really shine through.