Grease a 9-inch springform pan with butter, and set aside. Spread almonds evenly on a baking sheet, and bake until lightly toasted, 6 to 10 minutes. Transfer baking sheet to a wire rack; let almonds cool slightly, about 15 minutes. (Do not turn oven off.) Coarsely chop almonds and set aside.
Whisk together egg yolk, orange zest, almond extract, and vanilla extract in a small bowl, and set aside. Whisk together flour, cornmeal, and salt in a medium bowl. Using a pastry blender, cut butter into flour mixture. (Alternatively, rub butter pieces and flour mixture between your fingers until texture resembles a coarse meal.) Stir granulated sugar, brown sugar, and chopped almonds into flour mixture. Using your hands, gently work egg yolk mixture into flour mixture until combined. (Dough should be crumbly.) Loosely pack dough crumbles into an even layer in prepared springform pan.
Break the tart into large pieces. Pile on a platter with the Roasted Red Grapes and serve with the Champagne Sabayon.
Bake at 350°F until tart is golden brown, 25 to 35 minutes. Transfer pan to a wire rack, and let tart cool completely, about 1 hour. Remove sides of springform pan, and release tart. Break tart into large pieces and arrange on a platter alongside reserved roasted red grapes. Serve with Champagne sabayon.