Preheat the oven to 350°F (180°C). Butter two 8-inch (20-cm) cake pans. Line the bottoms with parchment rounds and butter the parchment. In a bowl, combine the almond meal, baking soda, and cinnamon. In a large bowl, whisk together the egg yolks, ⅓cup (2½oz/80g) of the sugar, the lemon zest, and lemon juice until smooth.
In a stand mixer, beat the egg whites on medium speed until frothy, about 2 minutes. Add the salt and cream of tartar and beat on medium-high speed until soft peaks form, about 2 minutes. Gradually add remaining ⅓cup (2½oz/80g) sugar, 1tablespoon at a time, beating until stiff peaks form.
Add the almond mixture to the egg yolk mixture, stirring with a wooden spoon. Fold in one-third of the beaten egg whites to lighten the batter. Then, in 2 additions, gently fold in the remaining whites just until incorporated. Spoon the batter into the prepared pans, dividing it evenly.
Bake the cakes until brown, springy to the touch, and a tester inserted into the center comes out clean, about 25 minutes. Transfer the cakes to wire racks. Using a small, sharp knife, cut around the cakes’ edges to loosen. Let cool completely in the pans.
Make the curd and frosting as directed.
To assemble the cake, put 1 cooled cake layer, rounded side up, on a platter. Using an offset or rubber spatula, evenly spread half of the reserved lemon curd on top of the cake. Spread about one-quarter of the frosting over the curd. Top with the second cake layer, rounded side up. Evenly spread the top of the cake with the remaining curd, then spread the remaining frosting over the top and sides of the cake. Place a few toothpicks in the top of the cake, and cover with plastic wrap. Refrigerate until ready to serve, up to 1 day.