Preheat oven to 350F and butter an 8-9” cake pan or springform. Line bottom with a buttered disc of parchment (‘cartouche’).
Melt butter and set aside to cool a bit. In a medium bowl whisk eggs, sugar and vanilla together until smooth and light colored. Whisk in the melted butter in two additions. Sift the cocoa, flour and salt into the egg mixture and fold gently to just combine. Transfer to the prepared pan, smooth out a bit and bake for 20 minutes until the top is just set. It will be sticky underneath. Turn off the oven, top the cake with the chocolate and toffee and return the pan to the oven for 4-5 minutes to slightly melt the topping. Let cool a bit before transferring to a serving plate.