1/4 cup light corn syrup
1 and 3/4 cups heavy cream, divided
1/2 stick unsalted butter
2/3 cup granulated sugar
1/2 cup unsweetened cocoa powder, plus more for serving
3 pints vanilla ice cream
18 graham crackers
1 tablespoons confectioners' sugar
Lightly coat an 8-inch square cake pan with cooking spray; line pan with plastic wrap, leaving a 6-inch overhang on two sides. Freeze pan at least 10 minutes. In a medium pot, combine corn syrup, 1/4 cup cream, butter, granulated sugar, cocoa, and a pinch of salt; bring to a boil, whisking, over medium-high. Remove from heat and let fudge sauce cool. Meanwhile, remove 1 pint of ice cream from freezer and let soften, 15 minutes.
Evenly press softened ice cream into pan. Top with a single layer of graham crackers, breaking to fit. Drizzle with 1/4 cup fudge sauce, spread and freeze 15 minutes. Repeat twice, omitting sauce on final layer (so graham crackers will be on top). Wrap in plastic and freeze until firm, 8 hours (or up to 3 days). Cover and refrigerate remaining fudge sauce.
To serve, you can either slice individual pieces (flipping so the graham cracker top is now the base), or invert pan onto a platter and remove plastic. Microwave fudge sauce for 30 seconds, or until pourable. Pour over cake and freeze to solidify, 5 minutes. In a large bowl, using a mixer, whip 1 and 1/2 cups cream and confectioners' sugar on high until stiff peaks form, 3 to 5 minutes. Dollop cake with whipped cream and dust with cocoa.