Preparation

Make the brittle

• Line a large rimmed baking sheet with parchment and set aside.
• Combine the sugar, corn syrup, salt, and 1/4 cup water in a small saucepan, and stir with a wooden spoon. Put the pan over medium-high heat, and cook without stirring until the mixture turns a deep amber color, about 15 minutes.
• Working quickly off the heat, add the sesame seeds, butter, baking soda, and vanilla, stirring until the butter melts.
• Pour the mixture onto the prepared sheet and use a small offset spatula or butter knife to spread the brittle as thinly as possible. (Work quickly; the brittle sets up fast.) Let cool to room temperature, about 2 hours.
• Break the brittle into small pieces, and store it in an airtight container at room temperature for up to a week.

Make the whipped cream

• Chill the bowl of a stand mixer and the whisk attachment (or a medium metal bowl and beaters from a hand mixer), about 15 minutes.
• Remove the bowl and whisk from the refrigerator, add the cream, and whip on medium speed until just thickened, about 1 minute.
• Add the confectioners’ sugar, tahini, vanilla, and salt, and whip the cream on medium-high speed until it holds stiff peaks, about 2 minutes.

Assemble the cake

• Coat a 9×5-inch loaf pan with cooking spray, and then line with plastic wrap. Add about 3/4 cup of the tahini cream to the bottom of the pan, and spread with the back of a spoon. Cover the cream with a layer of wafers, breaking them if necessary to create a solid layer of cookies. Add another 3/4 cup of the cream, and continue layering wafers and cream in this manner until you reach the top of the pan, ending with a final layer of cookies (you may have a little cream left over). Cover with plastic wrap, and refrigerate for 6 to 8 hours, preferably overnight.

Serve the cake

• Remove the cake from the refrigerator, remove the plastic wrap, and flip the cake onto a serving plate. Carefully lift off the pan, and then the plastic-wrap lining. Using a large serrated knife, cut the cake into slices. To get the cleanest slices, run the knife under hot water and then dry it between slices. Serve sprinkled with the brittle, and pass any remaining brittle at the table.