A large bowl should be wiped dry and degreased. Put the egg whites into it and let sit for 15 minutes, then add the lemon juice. Start beating with a whisk attachment on a stand mixer for about 2 minutes.
Add sugar and continue beating until stiff peaks appear. The pattern made with a whisk on the surface of beaten egg whites should be the same over and over. When using an electric mixer this will take maybe 4-5 min.
If you see that the peaks of whipped whites are too soft and do not hold their shape, continue beating until the meringue is as dense as possible. After that, you need to sift the cocoa powder along with the powdered sugar into a bowl.
Transfer the cocoa powder and powdered sugar mixture slowly into the bowl with egg white peaks. Be prepared for the meringue to become thinner at this stage. Therefore, it is important to do everything quickly. After adding the cocoa powder, stir gently but quickly several times until combined.
Transfer the meringue to a piping bag fitted with a star tip or any other. Pipe small cookie meringues onto a silicone mat or parchment paper. Place in preheated 160 degrees F oven for 1 1/2 - 2 hours until they are dried.
Drying time depends on the size of the meringue. The oven door can be opened at least one hour after the start of drying. To test doneness, break the meringue biscuits to test texture. They should be crispy and not bend, will continue stiffening up as they cool as well.
Store leftovers in a closed plastic food container and in a dry place.