Break up the 185g/6.5oz milk/semi-sweet chocolate & microwave in 30 second bursts, stirring well in-between each, until nearly melted. Then add in the cubed butter & mix well & then continue heating & mixing till all melted. Leave to cool while you gather the other ingredients. Alternatively, melt in a bowl over a pan of barely simmering water.
Weigh out the flour & cocoa through a sieve into another bowl (not the main mixing bowl).
Grease the baking tin & place baking paper/parchment inside it. See photo 1 above for how to cut your paper to fit. I usually hold in position with baking spray or Cake Lining Paste Next whisk the eggs & sugar with a hand/stand mixer for approximately 4 – 5 mins, on medium to high speed. It must become thick & creamy, paler in colour & increased in volume. This is the key to these gooey brownies. See Photo 2 above.
Gently fold in the melted chocolate/butter with a plastic/silicone baking spatula & do so in the shape of a number 8. Mixing too vigorously will undo all the whisking from earlier. Mix just until all mixed in and no more. See photo 3 above.
Next re-sieve the flour & cocoa mixture, into the batter. Mix it in again like before until no flour is visible, and scraping down the sides too.
Roughly chop the remaining 100g/3.5 oz dark/bitter chocolate & finally fold into the batter just till evenly distributed. See Photo 5 above.
Pour the brownie batter into the prepared pan & level off if required. See Photo 6 above. Leave to sit so that it will form a crust later. (Leave at least 10 mins). Heat up the oven to: 180°c/160°c Fan oven/350°f/Gas mark 4 while the brownie sits.
Bake for about 18 – 25 minutes, turning the tray if needed, after half-way through. The top will have formed the crust, (see photo 7 below). Testing with a wooden cocktail stick, & the outer edges produce a clean stick whist the centre is a little moist with just a few crumbs. This is what we want. Leave the brownies to sit to cool down in the tin for 5 minutes. Then transfer via the paper, to the cooling rack to cool down completely before cutting into 16 squares. See tips in notes for cutting
Store left-overs in an airtight container for a few days.