Heat oven to 350F and line an 8x8 pan with parchment.

Whisk butter and sugar, then beat in eggs and vanilla.

Stir in cocoa and flour until just combined and spread into the pan.

Dollop warmed hazelnut spread over the top and swirl with a knife.

Bake 30 to 32 minutes until set around the edges.

Cool completely before cutting to keep the swirls crisp.