Preheat oven to 170°C / 340°F (150°C fan) — note: lower than standard brownies. Line an 8×8 inch baking pan with parchment paper, leaving overhang on two sides.
Melt the butter gently and allow to cool 3 to 4 minutes. Pour the condensed milk into a large bowl. Add the warm melted butter and whisk until smooth and combined.
Add the eggs one at a time, whisking for 20 to 30 seconds after each. Add the vanilla extract with the second egg. Do not overwhisk.
Sift in the cocoa powder, then add the flour, salt, and baking powder. Fold with a spatula until just combined — 12 to 15 folds. Stop the moment you no longer see dry streaks. The batter will be very thick.
Fold in the chocolate chips if using, reserving a handful for the surface. Pour the batter into the prepared pan — spread with the spatula as it is too thick to self-level. Press reserved chips onto the surface.
Bake on the centre rack for 25 to 30 minutes. The edges should be set, the surface dry and matte, and a toothpick between chips should come out with moist crumbs. The pan will still wobble slightly — this is correct.
Cool at room temperature for at least 1 to 2 hours. Refrigerate for a minimum of 1 hour (overnight is best). Lift out using the parchment overhang. Cut into 16 squares with a warm, dry knife wiped clean between each cut.