In the bowl of a stand mixer with the whisk attachment (or use a hand mixer and a large bowl) whip the cream to medium peaks. Refrigerate.
Combine the yolks and sugar in a small metal bowl set over a saucepan with ½ inch of gently simmering water on the bottom. Whisk constantly until the sugar is dissolved and the yolks have lightened in color, about 2 minutes. Remove from heat.
Transfer about ⅓ of the whipped cream to a large bowl and pour in the yolk mixture. Stir together until well combined. Add half the remaining cream and fold gently with a rubber spatula until just incorporated. Repeat with remaining whipped cream.
In the mixing bowl that originally just contained the whipped cream (no need to clean it), combine about ½ cup of the whipped cream- yolk mixture and tahini until well combined. Carefully fold in the rest of the whipped cream-yolk mixture with a rubber spatula. Pour into a metal pan, cover with plastic wrap, and freeze for at least 4 hours and up to 3 days.
To make the Hot Fudge:
Mix the cream, brown sugar, cocoa powder, and corn syrup in a large saucepan. Bring to a boil over medium heat and cook, stirring frequently, for 30 seconds.
Remove from the heat and add the chocolate and butter, stirring until melted and smooth. Stir in the vanilla. Serve warm.
To make the Whipped Cream:
With an electric mixer, whip the cream until it begins to mound and hold its shape. Whisk in the sugar and vanilla, then whip until the cream forms soft peaks.
To assemble the Sundaes:
Place one scoop of the Semifreddo each in four sundae glasses. Top with 1 or 2 tablespoons Hot Fudge, a few banana slices and a few peanuts (if using). Top each with another scoop of Semifreddo, more hot fudge, banana slices and peanuts (if using), a dollop of whipped cream and a cherry. Enjoy immediately.
Recipe by Allons-Eat! at https://www.allonseat.com/tahini-semifreddo-sundaes/