Preheat the oven to 180°C
Grease and line an 8x8 inch baking tin with parchment paper. You can use a 9x5 inch loaf pan instead, but please note that baking times will vary.
Add the melted butter to a large mixing bowl. Whisk in a tbsp of oil.
Next, whisk in both the sugars until evenly combined.
Add the eggs and vanilla extract, beating them in vigorously until the mixture is paler in colour and frothy. Use a hand-mixer for this if possible.
Sift in the dry ingredients (cocoa powder, all-purpose flour and salt) and gently fold until just combined. Don't over-mix!
Fold in the chocolate chunks/ chips, reserving a few for the step below.
Pour the batter into your baking pan. Use an offset spatula/ the back of a spoon to smoothen out the surface and press the remaining chocolate chunks/ chips on top.
Bake the brownies at 180°C for 15- 18 minutes until the edges have set/ started to pull away from the tin but the centre is still very soft and hasn't fully set. As explained above, it's crucial that you don't over-bake these brownies. If you insert a toothpick in the centre, note that it will come out dirty with moist and fudgy crumbs.
Let the brownies cool down completely before slicing and serving with a sprinkling of sea salt. Enjoy!