Preheat oven to 350° Fahrenheit and grease an 8-inch cake pan.
Beat together the eggs and sugar with either a hand or stand mixer on high speed until pale and thick, about 5 minutes.
Add in the almond flour, lemon zest (if using) and baking powder. Beat for another 30 seconds to one minute, until batter is just combined.
Transfer the batter to your prepared cake pan and if you'd like, top with an even layer of sliced almonds (for extra crunch and decoration). Bake the cake 30-35 minutes, or until a toothpick inserted down the center of the cake comes out clean.
Remove cake from the pan and let cool completely before garnishing and serving.