To make the brownie layers, preheat the oven to 350? F. Grease and flour the edges of three 9-inch round cake pans and line the bottoms with parchment paper. Combine the butter and chocolate in a large heatproof bowl set over a few inches of simmering water. Continue heating until the butter and chocolate are melted and smooth, stirring occasionally with a spatula. Remove the bowl from the heat and set aside. Transfer the chocolate mixture to a large mixing bowl. (If your heatproof bowl is very large, transfer to a new bowl is not necessary.) Whisk in the sugar, eggs and vanilla extract until the mixture is smooth and glossy, about 1 minute. Stir in the flour and salt, and mix just until the dry ingredients are incorporated.
Divide the batter evenly between the prepared baking pans. Gently smooth the batter in the pans. Bake 20-25 minutes, until the brownies are just firm to the touch. Transfer to a wire rack and let cool in the pans 30 minutes. Then run a knife around the edge of each pan and invert the brownies onto the rack to cool completely.
To make the frosting, add the chilled heavy cream to the bowl of an electric mixer fitted with the whisk attachment. Whip on medium-high speed until medium-soft peaks form. Blend in the sugar, vanilla and espresso mixture, whipping just until combine. Continue to whip until stiff peaks form. Reserve and refrigerate 1 cup of the frosting for use in the filling. Set aside the rest of the frosting.
To make the filling, in the bowl of an electric mixer fitted with the paddle attachment, combine the mascarpone, espresso liquid, sugar, vanilla and salt. Beat on medium speed until well blended and fluffy, about 2 minutes. Gently fold in the reserved 1 cup of frosting with a spatula.
To assemble the torte, lay 1 of the brownie rounds to a large serving platter. Spread half of the filling mixture evenly over the top. Layer with another brownie round, and the remainder of the filling. Top with the final brownie layer. Cover the top and sides of the cake with the frosting. Garnish with the toffee bits to cover the top and sides of the cake. Use any remaining frosting to pipe decorative swirls around the top edge of the cake, if desired. Refrigerate until ready to serve.