
Preheat oven to 350F (175C) and lightly grease an 8x8” (20x20cm) square metal baking pan with butter (or line with parchment paper).
Combine eggs and sugar in a large mixing bowl and whisk vigorously until well combined and lightened in color (about 60 seconds).
Add Nutella and vanilla extract and whisk to combine.
Add melted butter and whisk again until well incorporated.
Separately whisk together flour, baking powder, and table salt.
Gently fold dry ingredients into wet then spread into prepared pan.
Drizzle additional Nutella over the surface and use a knife to swirl through the surface of the batter.
Transfer to center rack of 350F (175C) preheated oven and bake for 30-35 minutes, until a toothpick inserted in the center comes out clean or with a few fudgy crumbs.
Sprinkle with flaky sea salt while still warm.
Allow to cool before slicing and serving.