Preheat oven to 200C/180C Fan/Gas 6. Line 2/3 large baking sheets.
In a large bowl, or the bowl of a stand mixer, combine the butter and sugar until fully creamed (use an electric mixer or the paddle attachment on medium speed). Then add the egg and vanilla and mix again.
Add the flour, cocoa powder, baking soda and the pinch of salt! Stir together until just combined. Add in the chocolate chunks and chopped toffees and mix in.
Use a tablespoon or small ice cream scoop to scoop the batter onto the trays – approximately 4 cookies per tray – they spread a little. Wet your hands slightly and pat down and into rounds (not too squished, just no longer full balls). Bake 10-12 minutes, until just darkening at the edges.
Remove and let cool on the tray a little before moving to a wire rack. If misshapen as a result of baking, use a large round cookie cutter: Place around a still-warm cookie, nudging in any dough that's moved out of shape, and start spinning the cutter, it should bring the cookie with it, forcing it into shape!