Preheat your oven to 350°F (175°C). Line a baking sheet or jelly roll pan with parchment paper, leaving some overhang for easy removal.
In a large bowl, whisk together the sifted flour, cocoa powder, baking powder, salt, and sugar until well combined and free of lumps.
In a separate bowl, beat the eggs with vanilla extract and vegetable oil until smooth and slightly frothy, about 1-2 minutes.
Pour the wet ingredients into the dry mixture and gently fold until just combined, creating a thick, chocolatey batter.
Pour the batter into the prepared pan and spread it evenly with a spatula to form a thin layer. Tap the pan gently on the counter to remove air bubbles.
Bake for 12-15 minutes, or until the cake is slightly firm to the touch and a toothpick inserted in the center comes out clean.
Remove the cake from the oven and, while still warm, carefully lift it out of the pan using the parchment overhang. Place it on a clean kitchen towel and let it cool for about 10 minutes.
Mix the peanut butter with powdered sugar until smooth and creamy — this becomes your filling.
Unroll the slightly cooled cake gently if it has puffed up during baking. Spread the peanut butter filling evenly over the surface.
Carefully roll up the cake from one end to the other, using the parchment to help guide it into a tight spiral. Wrap it in plastic wrap and refrigerate for at least 1 hour for the flavors to meld and the roll to set.
Slice into thick rounds with a sharp knife and serve slices of the rich, swirled cake — enjoy the chewy crumb and gooey peanut butter core!
