Heat the oven to 180C/160C fan/gas 4. Butter and line a 20 x 20cm baking tin with baking parchment, leaving some overhang. Put the dark chocolate, butter and coffee in a heatproof bowl and set over a saucepan of barely simmering water ensuring the bowl doesn’t touch the water. Stir until smooth, then set aside to cool slightly. This can also be done in short bursts in the microwave, stirring between each burst. In a separate bowl, mix together the rye flour, cocoa powder and 1/2 tsp sea salt.

Using a stand mixer or electric hand whisk, beat together the eggs, sugar and vanilla for 5 mins until thick, pale, and doubled in volume. Gradually fold in the cooled melted chocolate mixture, being careful to retain as much air as possible. Gently sift the dry ingredients over the batter and fold in until just combined. Add two thirds of the chopped milk and white chocolate, and fold through briefly. Scrape the batter into the tin and smooth the top with a spatula, then scatter over the rest of the chocolate chunks and push them into the top slightly with the spatula. Bake for 30 mins or until the edges are set but the middle still has a slight wobble. Remove from the oven and scatter with a little flaky sea salt.

Let the brownies cool completely in the tin, then lift them out using the paper. If you want them super-gooey and soft in the middle, serve them at room temperature, but if you want them fudgy, chill in the fridge – either way they will cut into nine generous squares or 16 smaller pieces. Store in an airtight container at room temperature for up to three days, or in the fridge to go fudgy for up to a week.