Preheat the Oven:
Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan or line it with parchment paper for easy removal.

Make the Brownie Batter:
In a large bowl, whisk together the melted butter and sugar until smooth. Add the eggs, one at a time, whisking well after each addition. Stir in the vanilla extract.

Combine Dry Ingredients:
In a separate bowl, sift together the cocoa powder, flour, salt, and baking powder. Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Prepare the Strawberry Filling:
In a small saucepan over low heat, warm the strawberry jelly until it becomes slightly runny. If desired, stir in the cornstarch to thicken the jelly slightly.

Assemble the Brownies:
Pour half of the brownie batter into the prepared pan and spread evenly. Drizzle the warmed strawberry jelly over the batter, spreading it gently to create an even layer. Top with the remaining brownie batter, spreading carefully to cover the jelly layer.

Bake:
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (avoid overbaking!).

Cool and Serve:
Allow the brownies to cool completely in the pan before cutting into squares. Serve and enjoy the gooey, fruity surprise in each bite!