Spray a loaf pan with nonstick cooking spray. Carefully line with plastic wrap, smoothing it the best you can.
Process cookies in a food processor. Combine crumbs and butter until thoroughly mixed. Press crumb mixture evenly in bottom of loaf pan and put in the freezer while you make the custard.
Put some water in the bottom part of a double boiler and bring to a simmer.
In the top of the double boiler, whisk together the egg yolks, 1/2 cup of the sugar and 1 teaspoon of vanilla. Place top of double boiler over bottom part with simmering water. Cook, whisking constantly, until egg mixture temp reaches 160 degrees. This should take about 5 minutes or so. The egg yolks will thicken somewhat and fall off the whisk in ribbons.
Remove from heat. Whisk in the hazelnut spread and let cool to room temperature. Whisk occassionally while cooling. You can let it cool in the refrigerator or use an ice bath.
In the bowl of a stand mixer, whip the heavy cream on medium until it starts to thicken. Increase speed and beat until soft peaks begin to form, then gradually add the remaining 1/4 cup of sugar. Beat until stiff peaks form.
Whisk 1/4 of whipped cream into yolk mixture, then gently fold in the rest until no white streaks remain. Scrape mixture to loaf pan. Tap it on counter to level, then cover top of pan with plastic wrap. Freeze at least 24 hours.
Remove from freezer just prior to serving. Slice and serve.