Preheat oven to 350°F.
Prepare 3 Cake Pans (8"x8"x2"): use bottom of a cake pan to measure/trace 3 circles on parchment paper; cut out the circles and set aside. Spray the inside of the pans with canola oil. Place parchment circles on the bottom of each pan and spray over top with butter flavored oil. Set aside.
Prepare & Bake Cakes: in three separate bowls, prepare each cake mix. For vanilla cake, mix together 2 eggs, 1 stick of melted butter, and 1 cup milk; add dry mix and whisk until batter is smooth; pour into cake pan. For brownies, whisk together 2 eggs with 1 stick of melted butter; add dry mix and stir until moistened; pour into cake pan. For blondies, whisk together 1 egg with 1 stick of melted butter; add dry mix and stir until moistened; pour into cake pan. Bake all three cakes per directions on package (note: the vanilla cake takes about 10 minutes longer, so make this one first). Remove cakes and let cool (in pans) for 10 minutes. After 10 minutes, run a knife around the inside of each cake pan, flip pans upside down and ease cakes out onto a cooling rack to cool completely, about 40-50 minutes.
Prepare Frosting: while cakes are cooling, make buttercream frosting. Add butter to stand mixer (a hand mixer will work too) fitted with a paddle; beat at medium speed until creamy, about 2 minutes. Add sugar, cream, and vanilla; mix at low speed until just incorporated. Increase the speed to high and mix until smooth and fluffy, about 3 to 5 minutes. If it becomes too thick, add a little more cream. If it’s too thin, add more powdered sugar.
Assemble Cake: place a dollop of buttercream on cake stand or platter to anchor, place brownie layer on top. Pipe or spread buttercream frosting along the edges of the cake; fill in center with ¼ jar of caramel sauce. Top with blondie cake and repeat buttercream and caramel step. Top with vanilla cake layer; as lightly as you like frost sides of the entire cake with remaining buttercream.
Decorate Cake: warm the jar of fudge sauce for 10 seconds in the microwave and pour ½ over the top of the cake and allow to flow down the sides in long drips. Decorate top of the cakes with brownie petites and cookies.