¼ cup light corn syrup
1¾ cups heavy cream, divided
½ stick unsalted butter
⅔ cup granulated sugar
½ cup unsweetened cocoa powder, plus more for serving
3 pints vanilla ice cream
18 graham crackers
1 tablespoon confectioners' sugar
Unsweetened cocoa powder
1. 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.
2. In a medium pot, combine corn syrup, 1/4 cup cream, butter, granulated sugar, cocoa, and pinch of salt; bring to a boil, whisking, over medium-high. Remove from heat and let fudge sauce cool. Meanwhile, remove 1 pint ice cream from freezer and let soften, about 15 minutes.
3. 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 and freeze 15 minutes. Repeat twice, omitting sauce on final layer. Wrap in plastic and freeze until firm, 8 hours (or up to 3 days). Cover and refrigerate remaining fudge sauce.
4. To serve, invert pan and run under tepid water to loosen cake. Invert onto a platter and remove plastic. Microwave fudge sauce in 30-second increments until pourable. Pour over cake and freeze to solidify, 5 minutes.
5. In a large bowl, using a mixer, whip 1 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.