How to Make It
Fill a large bowl with ice and water. In a medium saucepan, combine the maple syrup with the vanilla bean and seeds and bring to a boil. Cook over moderate heat, stirring occasionally, until extremely fragrant, chocolate brown in color and thickened, 3 to 5 minutes. Remove the saucepan from the heat. Whisking constantly, slowly drizzle in the cream. Scrape the maple cream into a medium metal bowl and set it in the ice bath; stir occasionally until very cold, about 30 minutes. Discard the vanilla bean. Refrigerate the maple cream.
Meanwhile, in the bowl of a stand mixer fitted with the whisk, beat the egg, egg yolks and salt at medium speed until the mixture is light and fluffy, about 3 minutes.
In a small saucepan with a candy thermometer attached, combine the maple sugar with 1 tablespoon of water and bring to a boil. Cook over moderate heat, swirling the pan occasionally, until the syrup is very foamy and reaches 235°. With the mixer at low speed, slowly drizzle the hot syrup down one side of the egg mixture and beat until the side of the bowl is cool to the touch, about 5 minutes.
Using a hand mixer, beat the chilled maple cream at medium speed until firm. Fold the maple cream into the egg mixture. Scrape the semifreddo into a 9-by-13-inch baking pan in an even layer. Freeze until firm, about 4 hours or overnight.
Serve the maple semifreddo in tall glasses layered with the candied pecans.