Evenly cover an 8-inch square cake saucepan with cooking spray, line pan with plastic cover, parting a 6-inch overhang on two sides. Freeze pan for 10 minutes.
In a standard pot, merge corn syrup, 1/4 cup cream, butter, crushed sugar, cocoa, and pinch of salt; take to a boil, whipping, over medium-high. Eradicate from heat and let fudge sauce cool. In the meantime, eradicate 1 pint ice cream from freezer and let soften, about 15 minutes.
Lightly press soften ice cream into pan. Top with a sole layer of graham crackers, flouting to fit. Sprinkle with 1/4 cup fudge sauce and freeze 15 minutes. Replicate twice, skipping sauce on final layer. Cover in plastic and freeze until stiff, 8 hours. Wrap and cool remaining fudge sauce.
To dish up, invert pan and run under moderate water to loosen cake. Invert onto a platter and eradicate plastic. Microwave fudge sauce in 30-second rise until pourable. Decant over cake and freeze to solidify, 5 minutes.
In a huge bowl, with a mixer, whisk 1 1/2 cups cream and confectioners' sugar on high until stiff peaks form, 3 to 5 minutes. Squirt cake with whisked cream and dust with cocoa. Serve instantly.