Coat an 8-inch-square baking dish with cooking spray and line with plastic wrap, leaving an overhang on all sides. Combine the vanilla wafers and pretzels in a large zip-top bag; crush with a rolling pin until you have coarse crumbs with some large pieces. Transfer to a medium bowl.
Beat 2 3/4 cups heavy cream in a large bowl with a mixer on medium speed until soft peaks form, about 2 minutes. Sprinkle in 1/2 cup confectioners' sugar and continue beating until just stiff, about 1 more minute. Mix 1/2 cup of the whipped cream and 3/4 cup dulce de leche in a small bowl, then fold the mixture back into the remaining whipped cream until combined. Mix 2 cups of the dulce de leche whipped cream into the cookie-pretzel crumbs.
Spread half of the remaining dulce de leche whipped cream in an even layer in the prepared dish. Drop spoonfuls of the cookie-pretzel mixture on top and gently smooth with an offset spatula or your fingers. Top with the remaining dulce de leche whipped cream and smooth the top. Cover with plastic wrap and refrigerate at least 8 hours or overnight.
Uncover and invert onto a platter; remove the plastic. Beat the remaining 1 1/4 cups heavy cream with a mixer on medium speed until soft peaks form, about 2 minutes. Sprinkle in the remaining 1/4 cup confectioners' sugar and beat until stiff peaks form, about 2 more minutes. Spread on top of the cake. Put the remaining 1/4 cup dulce de leche in a microwave-safe measuring cup; microwave until pourable, 20 to 30 seconds. Drizzle over the cake.
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