Preheat the oven to 375°F (190°C). Grease an 8-inch round cake pan and line the bottom with parchment paper.
In a microwave-safe bowl, melt the chocolate chips and butter together in 30-second intervals, stirring until smooth.
Stir in the sugar, salt, and vanilla extract. Add the eggs one at a time, mixing well after each.
Sift in the cocoa powder and stir until just combined.
Pour the batter into the prepared pan and smooth the top.
Warm the peanut butter slightly, then drizzle it over the top of the cake batter. Use a knife or toothpick to swirl it into the batter.
Bake for 20–25 minutes, until the center is just set.
Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.