Step 1: Preheat and Prepare Pan
Preheat oven to 350°F (175°C). Line an 8×8-inch square baking pan with parchment paper, leaving overhang on two sides for easy removal. Grease the parchment lightly with butter or cooking spray. Set aside.

Step 2: Melt Chocolate and Butter
In a medium saucepan over low heat, melt the butter and chopped chocolate together, stirring constantly until smooth and completely melted. Remove from heat and let cool for 5 minutes. (Alternatively, melt in microwave in 30-second intervals, stirring between each.)

Step 3: Prepare Raspberry Swirl
In a small saucepan or microwave-safe bowl, combine raspberries, powdered sugar, and lemon juice (if using). Heat gently until raspberries break down, about 3-4 minutes on stovetop or 1 minute in microwave. Mash with a fork or potato masher until you have a chunky sauce. If you prefer seedless, strain through fine-mesh strainer (optional). Set aside to cool slightly.

Step 4: Mix Brownie Batter
Add sugar to the melted chocolate mixture and whisk vigorously for about 1 minute until smooth and glossy. Add eggs one at a time, whisking well after each addition. The batter should become thick and shiny. Add vanilla extract and whisk until combined.

Step 5: Add Dry Ingredients
In a small bowl, whisk together flour, cocoa powder, salt, and baking powder (if using). Add the dry ingredients to the chocolate mixture. Using a rubber spatula, fold gently just until no flour streaks remain. Don’t overmix! The batter should be thick and fudgy.

Step 6: Layer and Swirl
Pour brownie batter into prepared pan, spreading evenly with a spatula to reach all corners. Drop spoonfuls of the raspberry mixture over the top of the brownie batter (about 8-10 dollops scattered across the surface). Use a butter knife, toothpick, or skewer to swirl the raspberry through the chocolate batter, creating a marble pattern. Don’t over-swirl – you want distinct ribbons of raspberry, not a muddy mixture. 5-6 swirls through each section creates beautiful effect.

Step 7: Bake
Bake for 30-35 minutes. Brownies are done when edges look set and center has just a slight jiggle. A toothpick inserted near the center should come out with moist crumbs (not wet batter, but not completely clean). Don’t overbake! For 8×8 pan, start checking at 30 minutes. For 9×9 pan, check at 28 minutes. Remember, brownies continue cooking from residual heat.

Step 8: Cool Completely
Remove from oven and place pan on a wire rack. Let brownies cool completely in the pan for at least 1 hour (or 2-3 hours for cleanest cuts). Once cool, use the parchment overhang to lift brownies out of pan. Cut into 16 squares with a sharp knife, wiping blade clean between cuts.

Step 9: Serve and Enjoy
Serve at room temperature. These are delicious on their own or with a scoop of vanilla ice cream. Store any leftovers covered at room temperature or refrigerated.