Preheat your oven to 350°F and line an 8x8-inch baking pan with parchment paper.

To make the base, cream together butter, brown sugar, vanilla and salt in a medium bowl with a rubber spatula or wooden spoon until smooth, creamy and paste-like. Sift in flour and fold it through until combined. Turn dough out into prepared pan and press it down evenly to form a flat layer. Use a piece of parchment or waxed paper on top of the crust to help smooth it out or run the back of a spoon over the surface. Bake for 16-20 minutes, until lightly golden. Transfer to a wire rack to cool slightly.

To make ganache, first place chocolate in a medium heatproof bowl and set aside. Be sure to use dark chocolate with minimum 70% cocoa or the ganache will be too soft.

Place sugar in an even layer over the base of a 1 or 2-quart heavy-bottomed stainless steel saucepan. Make sure the pan you choose is at least 8-inches in diameter. Place the pan over medium-high heat and move the sugar slowly from side to side with a heat-resistant spatula or wooden spoon until it begins to melt. Once you see that it is liquifing into a syrup, continue to move it gently until it starts to turn golden, pushing sugar from the sides to the center and vise versa. Reduce heat slightly as it will colour quickly at this point and continue to cook while stirring until it all melts into a liquid and turns a dark amber colour. Remove from heat and immediately stir in butter. Stir in cream until smooth. Stir in salt and peppermint extract. If you can still feel undissolved caramel in the corners of the pan as you stir, place over low heat and stir until melted. Let it cool for 30 seconds before pouring the caramel over the chocolate in the bowl. Stir gently until completely smooth and glossy.

Pour the caramel over the still-warm baked cookie crust. Tilt the pan to coax it into the corners to form an even layer. Refrigerate for at least 2 hours before slicing and dust with cocoa powder before serving.