Preheat oven to 350°F. Grease a 9-inch springform pan and line the bottom with parchment.
Whisk almond flour, baking powder, and salt in a bowl; set aside.
Beat the eggs and sugar in a large bowl until thick and pale, about 2–3 minutes.
Whisk in melted butter, lemon zest, lemon juice, vanilla, and almond extract until smooth.
Fold the dry mixture into the wet just until combined. Spread batter into the pan; tap to level. Sprinkle sliced almonds if using.
Bake 32–38 minutes, until golden and a tester comes out clean or with a few moist crumbs.
Cool 10 minutes, release the ring, and cool completely. Dust with powdered sugar, slice, and serve.