Place the gelatin leaves in a medium bowl. Add 1/2 cup water. Let stand until softened, about 10 minutes.
Prepare an ice bath; set aside. In a large saucepan, melt the butter over medium heat. Stir in the flour, and cook, stirring for 1 minute. Whisk in the stock, and bring to a boil, whisking constantly. Reduce to a simmer, cook until thickened, about 2 minutes.
Add softened gelatin and water, and stir until the gelatin is completely dissolved. Remove from heat, and stir in the cream and Cognac. Season with salt and pepper. Transfer to the ice bath, and let stand, whisking frequently, until room temperature.
Line a baking sheet with a wire rack. Place ham on rack, with the bone on top. Ladle enough sauce over ham to cover, creating an even layer. Chill until set, about 10 minutes. Repeat process.
Decorate as desired with carrots, beets, and leeks, pressing the decorations lightly into the sauce to adhere.
Ladle the aspic over the ham, creating an even layer. Let stand until set. Repeat process. Serve immediately or keep refrigerated up to overnight. The ham will not be as shiny if refrigerated overnight.