1. Preheat the oven to 350ºF (180ºC). Butter an 8-inch (20cm) round cake pan, or similar-sized vessel. (See headnote.) Line the bottom with a circle of parchment paper, if you wish.
2. In a small bowl, whisk together the almond flour, all-purpose flour, and salt.
3. In the bowl of a stand mixer fitted with the paddle attachment (or by hand in a bowl, with a spatula), beat the butter, sugar, and lemon zest until light and fluffy, about 3 minutes on medium-high speed. On low speed, stir in the eggs one at a time, then the juice from half a lemon, stopping the mixer to scrape down the sides of the mixer bowl after the addition of each egg.
4. Stir in the flour mixture until completely incorporated, then scrape the batter into the prepared cake pan. Smooth the top.
5. Bake the cake until the center feels set; a toothpick inserted into the center will come out clean, about 40 minutes. Don’t worry if the sides (or top) get too dark; that’s normal and the cake will be fine.
6. Remove from the oven and let cool 5 minutes. Sprinkle the remaining lemon juice over the cake, and the amaretto. Let cool before slicing.