1 Heat the oven to 350F and prepare the pan: Line an 8×8 baking pan with a sling of parchment, long enough so that the parchment hangs over the sides of the pan. Coat the exposed sides of the pan with cooking spray (also, a little cooking spray under the parchment will help it stick in place, if needed).
2 Combine the dry ingredients: In a bowl, whisk together the flour, baking powder, and salt. Set aside.
3 Melt the chocolate and butter: In a medium saucepan over medium-low heat, melt the butter. It should not brown at the edges.
Add the chocolate and let it melt, stirring occasionally with a heatproof spatula, until the mixture is smooth.
4 Set the pan on a cooling rack to cool until you can touch the bottom of with your hand comfortably.
5 Make the batter: Using a rubber spatula, stir the sugar into the chocolate mixture until it combined. The mixture may look crumbly or smooth after mixing; either is fine.
Add the eggs, one at a time, followed by the vanilla. Add the flour mixture, and stir until no more white patches show. Stir in the chips until well distributed.
6 Pour the batter into the pan. Use a spatula to spread it to the edges of the pan and smooth the top. Tap the pan once or twice hard on the counter to settle any air pockets; you can also poke the air pockets with the tip of a paring knife.
7 Bake the brownies: Transfer the pan to the oven and bake for 25 to 30 minutes, or until the top is slightly puffed and just set. If in doubt, it’s better to under-bake these brownies a touch rather than over-bake. (The toothpick test doesn’t work well for these brownies because the chocolate chips get melty!)
8 Cool the brownies completely: Set the pan on a wire rack until completely cooled.
9 Once cool, cover the pan and refrigerate for several hours or until firm and cold. This makes the brownies easier to slice cleanly.
If you’re eager for brownies and can’t wait for them to chill, skip this chilling step and serve them warm or just-cooled! They will be a little crumbly when you cut them.)
6 Cut the squares: Run a thin knife around the edge of the pan to make sure the block of brownies is loose, then lift from the pan using the parchment paper sling. On a cutting board, slice the brownies into a 5×5 grid (to make 25 squares) or into a 6×6 grid (to make 36 mini-brownies).
Brownies can be served right away. Leftovers can be stored in an airtight container at room temperature for at least a week. (They will keep for longer, but may start getting dry and crumbly after a while.)