Nutella and Biscoff are both European dessert spreads that can usually be found at most grocery stores here in the states. If you're coming up empty handed, you can procure some online.
In a medium saucepan over medium-low heat, melt butter and chocolate; stir until smooth. Transfer to a large bowl and whisk in sugar. Add eggs, one at a time, whisking well after each addition. Whisk in vanilla, flour, salt and baking soda.
Pour into three greased and floured 9-inch round baking pans. Bake for 23-25 minutes. Cool for 10 minutes in the pans; remove from pans to a wire rack to cool completely. Wrap in plastic wrap or store in air-tight containers until needed.
Malted Nutella and Biscoff layers
Line two round 9-inch pans with plastic wrap (the same ones used to make the brownie layers, if possible).
Make the Biscoff layer: Mix together 3 cups of the powdered sugar, 1 cup Biscoff and 4 tablespoons melted butter in a standing mixer fitted with the paddle attachment. Mixture will be crumbly. Add 2 to 3 tablespoons of cream 1 tablespoon at a time until the mixture becomes consistent workable dough. It should not be crumbly or stick to your fingers. The mixture should hold together easily when a small amount is squeezed tight in the palm of your hand. Press the dough evenly into one of the lined pans. Cover and chill for 30 minutes.
Make the malted Nutella layer: Mix together 3 cups of the powdered sugar, 1 cup Nutella, malted milk and remaining 4 tablespoons melted butter in a standing mixer fitted with the paddle attachment. Mixture will be crumbly. Add 2 to 3 tablespoons of cream 1 tablespoon at a time until the mixture becomes consistent workable dough. It should not be crumbly or stick to your fingers. The mixture should hold together easily when a small amount is squeezed tight in the palm of your hand. Press the dough evenly into the remaining lined pan. Cover and chill for 30 minutes.
Salted Biscoff frosting
Combine the butter and Biscoff in the bowl of an electric mixer fitted with the whip attachment. Beat until fluffy. Add the powdered sugar and mix again. Add cream 1 tablespoon at a time until the mixture is thick and fluffy; add the salt and whip again. Scrape down the bowl and beat for 2 minutes longer, or until the salt is well dispersed throughout the batter.
Nutella Frosting
Beat the butter and Nutella in the bowl of an electric mixer fitted with the whip attachment. Beat until fluffy. Add the powdered sugar and mix again. Add cream 1 tablespoon at a time, if needed. Transfer the frosting to a large piping bag fitted with a large closed star piping tip.
Assembly
Place a brownie layer on a serving plate or cake stand. Turn out the Nutella candy layer on top of the brownie; top with another brownie layer. Turn out the Biscoff layer onto the brownie layer and top with the final brownie layer. Frost the entire cake with the salted Biscoff frosting, using an offset spatula to swirl the icing. Pipe 12 to 14 rosettes of Nutella frosting on the top edge of the cake. Garnish each with jumbo malt ball. Place the cake on a baking sheet and gently toss pulverized cookies onto the bottom edge of the cake, allowing the excess to fall onto the baking sheet. Finally garnish the top of the cake with Maldon flake salt.