Make the genoise: Preheat oven to 400 degrees. Coat 2 rimmed baking sheets with cooking spray, line each with parchment, then spray parchment. Combine cake flour, almond flour, and salt in a medium bowl.
Whisk eggs, yolks, and granulated sugar in a mixer bowl set over a pot of simmering water until sugar dissolves and mixture is warm to the touch, about 2 minutes. Attach bowl to mixer, and whisk on medium speed for 2 minutes. Raise speed to high, and whisk until mixture is pale and thick, 4 to 5 minutes. Sift flour mixture over egg mixture, and gently fold. When almost incorporated, slowly add melted butter, and fold.
Divide batter between baking sheets, spread evenly, and bake until golden brown and springy to the touch, 8 to 10 minutes. Let cool. Run a knife around edges of pans, and invert onto a clean surface; discard parchment. Using an 8-by-3-inch cake ring, cut out 4 circles. Choose the 3 thickest cake circles, and reserve the fourth for another use.
Make the pastry cream: Bring milk, vanilla seeds, salt, and 1/4 cup granulated sugar to a boil in a medium saucepan. Meanwhile, in a medium bowl, whisk yolks, cornstarch, and remaining 1/4 cup granulated sugar to combine. When milk begins to boil, remove from heat. Slowly ladle half the milk into yolk mixture, whisking constantly. Pour mixture into saucepan, place over medium heat, and cook, whisking constantly, until mixture has thickened, about 30 seconds. Remove from heat, and whisk in cold butter. Strain into a medium bowl. Press plastic wrap directly onto surface, and let cool. Refrigerate for at least 2 hours (or overnight).
Make the simple syrup: Bring granulated sugar and water to a boil in a small saucepan, stirring until sugar dissolves. Remove from heat, and stir in Armagnac if using. Let cool. (Syrup will keep, covered and refrigerated, for up to 1 month.)
To assemble the cake: Place the cake ring on a 9-inch cardboard circle. Place 1 cake circle in ring, and lightly brush top with syrup. Using a small offset spatula, spread jam on cake layer. Whisk 2 1/2 cups cream until stiff peaks form. Cover jam with 1/2 cup whipped cream. Top with another cake circle, and brush top with syrup. Cover this circle with pastry cream. Place final cake circle on top, pressing down gently, and brush top with syrup. Spoon remaining whipped cream over cake, and smooth. Cover, and refrigerate for at least 4 hours (or preferably overnight).
Whisk remaining 1/2 cup cream in a medium bowl until stiff peaks form. Remove cake ring, and coat sides of cake with whipped cream. Refrigerate cake until cold, about 30 minutes.
Knead a golf-ball-size piece of marzipan with a couple of drops of blue food coloring, adding more drops as needed to reach desired color (this piece should be a few shades darker than the final). Knead half the colored marzipan into remaining marzipan until color is evenly distributed. (Add remaining half if needed to reach desired color.) On a surface dusted with confectioners' sugar, roll out marzipan to a 14-inch circle, about 1/8 inch thick. Place a doily in the center of marzipan. With the rolling pin, slowly but firmly roll over doily to create an imprint. Gently remove doily. Rub confectioners' sugar into marzipan with your fingertips to fill imprint. Place marzipan over cake, smooth sides, and trim bottom edges. Refrigerate for up to 3 days.