In a food processor, combine the all-purpose flour, almond flour, sugar, cocoa and salt; pulse to blend. Add the butter; pulse until the mixture resembles coarse meal. Add the egg; process until the dough just comes together. Turn the dough out onto a sheet of plastic, flatten into a disk and chill for 30 minutes.
Preheat the oven to 350°. Butter an 11-inch fluted tart pan with a removable bottom. On a lightly floured work surface, roll the pastry out to a 14-inch round 1/4 inch thick. Fit the pastry into the tart pan. Run the rolling pin across the rim of the pan to cut off any overhanging dough. Patch any tears in the pastry with scraps. Line the pastry with foil and fill with pie weights or dried beans. Bake in the center of the oven for 30 minutes, or until the crust is nearly dry. Remove the foil and bake the crust for 15 minutes longer, or until firm. Let cool. Lower the oven temperature to 300°.
Set a fine sieve over a medium bowl. In a medium saucepan, combine 1/2 cup plus 1 tablespoon of the sugar with the lemon juice and bring to a simmer, stirring until the sugar dissolves. In another medium bowl, beat the egg yolks with the whole egg and the remaining 1/2 cup plus 1 tablespoon of sugar. Whisk half of the hot lemon syrup into the egg mixture; add the mixture to the saucepan and cook over moderate heat, stirring constantly, until the curd just comes to a boil, about 3 minutes; it should be the consistency of sour cream. Strain the curd through the fine sieve and let cool to room temperature, then whisk in the softened butter until the curd is light and fluffy.
Pour the curd into the crust and bake for 20 minutes, or just until set. Cool the tart on a wire rack, then refrigerate until chilled. Unmold the tart and garnish it with chocolate shavings. Transfer the tart to a large plate and serve.
The baked tart can be refrigerated overnight.
Almond flour is available at specialty markets. Or finely grind 3/4 cup (3 ounces) of blanched almonds.