Preheat your oven to 375°F (190°C). Grease an 8 x 11–inch (20 x 28–cm) casserole dish and line with parchment paper, letting the excess fall over the sides.
Put the butter in a medium saucepan and melt over medium heat. Once melted, crank up the heat to medium high. Continue stirring and look for small golden bits that will start to settle on the bottom of the pan. It will smell deliciously nutty and caramel-y. This should take around 3 to 5 minutes. Once this happens, take it off the heat and pour into a medium, heat-safe bowl to cool a bit.
When cooled slightly, whisk in the muscovado, granulated sugar, oil, eggs, egg yolk, milk and vanilla until thoroughly blended.
In another medium bowl, whisk together the bread flour, all-purpose flour, baking powder, baking soda and sea salt. Add this to the butter mixture and stir until barely blended. Add in the chocolate chunks and continue to blend until just combined and the chocolate is evenly distributed.
Pour the mixture into the prepared baking dish. Dampen your hands and gently press down so that it covers the bottom of the dish. Bake in the center of the oven for 25 minutes. It will continue to cook and firm up as it cools. Sprinkle with sea salt flakes, if desired. Let cool completely before cutting. These are great the same day as they’re made and equally dazzling the next.