Preheat the oven to 180 C/350 F. Line an 8 x 8-inch pan with parchment paper, and set aside.: As the oven warms to 180 C/350 F , you will notice the faint hum and the initial dry, warm air that signals readiness, which is important because an accurate starting temp ensures even baking. Lining an 8 x 8-inch pan with parchment creates lift and prevents sticking, and the paper edges give you handles for removing the brownies. If you skip lining, you risk tearing the edges when extracting the bars, so never underestimate this small step. Avoid using a different pan size unless you adjust bake time, because a larger pan will thin the batter and overbake it, while a smaller pan will lengthen baking and could undercook the center.
In a microwave-safe bowl, add your chopped chocolate. Melt your chocolate. Let sit for 1-2 minutes, before adding your yogurt and sugar. Whisk until smooth and transfer to a large mixing bowl.: When you melt the chopped chocolate , the scent becomes noticeably warm and cocoa rich, a cue that the base flavor is forming. Letting it rest for 1 to 2 minutes allows residual heat to finish melting, preventing scorching when you stir in cooler ingredients. Whisking until glossy ensures the emulsion is smooth, and once you add the yogurt and sugar , the mixture should become satin like. If you blitz the chocolate too aggressively in the microwave, it can seize, becoming grainy, so always heat in short bursts and stir frequently. A common mistake is adding cold yogurt right away; allowing it to come slightly closer to room temperature helps the mixture stay uniform and prevents curdling.
Add the flour, cocoa powder, and baking soda, and mix until just combined. Add your milk and mix well, until fully incorporated. Fold through your chocolate chips, if using them.: As you sprinkle in the flour , cocoa powder , and baking soda , the batter will darken and thicken, and you want to stir until there are no streaks of dry ingredients visible. Overmixing is the enemy here because it develops gluten and yields a cakier texture, so stop as soon as unified. Adding the milk adjusts the batter to a glossy, pipeable consistency; you want it dense but pourable, not runny. Folding in chocolate chips creates melty islands that burst when you bite into them, and doing this gently keeps them from sinking or bleeding. A typical error is vigorous beating at this stage, which aerates the batter and changes the final crumb, so use gentle strokes.
Transfer to the lined pan and bake the brownies for 25-30 minutes, or until a skewer comes out just clean. Remove from the oven and let cool in the pan for 10 minutes, before carefully transferring to a wire rack to cool completely.: Sliding the batter into the prepared pan, you will notice a glossy top that puffs slightly as it bakes, and the kitchen will fill with an intense chocolate aroma that tells you things are progressing. Baking for 25 to 30 minutes aims for a set exterior and a tender interior, and checking with a skewer that comes out just clean, or with a few moist crumbs, ensures the center remains fudgy rather than dry. Letting the brownies rest in the pan for 10 minutes allows internal steam to redistribute, making it easier to remove them intact. Transferring too soon can cause the squares to fall apart, and leaving them in the pan too long may trap condensation, softening the crust. When slicing, use a sharp knife and wipe between cuts for neat edges.