Mix almond flour and powdered sugar in a food processor and blitz it 10-15 times to make the flour even finer.
Sift the almond/powdered sugar mixture into a large bowl and set aside.
In the bowl of a stand mixer with a whisk attachment, add the egg whites, cream of tartar and salt. Mix on low one minute, then medium another minute, until you reach high speed. Slowly stream in the granulated sugar. Continue mixing until the egg whites form stiff peaks.
Once you've achieved stiff peaks, add in the food coloring and vanilla and mix on high again about 30 seconds to another minute.
Using a spatula fold in one third of your egg white mixture into the dry ingredients, being careful not to overwork the whites. You don't want them to lose their air bubbles. Next fold in another third and finally the last bit of egg whites. At this point your batter should look and move like lava. The true test is to see if you can use your spatula to pick up some batter and draw a figure 8 in the bowl, without the stream of batter breaking.
Place the batter in a piping bag fitted with a #4 round piping tube (or any similar round piping tube) and pipe circles onto a silpat lined baking sheet. I use a baking sheet that has circles already printed on it for reference but if you don't have one, use a parchment lined baking sheet.
Tap the baking sheet on the counter a few times to get rid of bubbles. Then let the macs sit for 15-30 minutes while the oven preheats.
Bake at 300 degrees F. for 12-15 minutes. Rotate the tray around minute 7 for best results. Allow the tray to cool completely before removing from the oven.