Place the butter and chocolate in a heatproof bowl fitted over a saucepan with some simmering water and melt until smooth. Cool slightly until just warm to the touch.
Preheat the oven to 350 degrees F. Butter the inside of a 9-inch springform pan.
Place the egg whites in a medium bowl with the salt. Place the yolks in a slightly larger bowl with the granulated sugar. Using a handheld mixer, beat the whites on medium speed until stiff peaks form, about 2 minutes. Transfer the mixer to the bowl with the yolks and beat on medium speed until light, pale and fluffy, about 1 1/2 minutes. Using a rubber spatula, stir the chocolate mixture, almond flour and lemon zest and juice into the yolk mixture. In 2 batches, fold the whipped whites into the chocolate mixture. Pour the batter into the prepared pan and bake until puffed, golden brown and a toothpick inserted in the center comes out with just a few moist crumbs, about 45 minutes. Cool completely on a wire rack.
Run a thin knife around the edge of the pan to release the cake. Remove the outside ring and dust the top with confectioners' sugar. Serve with soft whipped cream, if desired.