Preheat the oven to 350°F (175°C) and grease a 9x9 inch baking pan.

In a large bowl, whisk together melted butter, sugar, cocoa powder, and salt until smooth.

Add the vanilla extract and eggs one at a time, mixing well after each addition.

Fold in the flour until just combined, being careful not to overmix.

Pour the batter into the prepared baking pan and spread evenly.

Drop spoonfuls of raspberry jam over the batter, then use a knife to swirl it gently.

Bake for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.

Let cool completely before slicing and serving.