Step 1

Position a rack in the center of the oven and preheat to 375ºF. Line an 8x8-inch baking pan with parchment on the bottom and sides.

Step 2

In a large bowl, whisk together the egg and sugar. Whisk in the melted butter or oil and vanilla.

Step 3

Sift in the flours, baking powder, and fine salt and stir until just combined.

Step 4

Stir in the chocolate and nuts (if using).

Step 5

Spread the batter in the prepared pan and top with more hazelnuts, chocolate, and a few pinches of flaky salt. It will seem like not enough batter, but it will puff up as it bakes.

Step 6

Bake until a toothpick inserted near the center comes out with moist crumbs, 18-22 min. Take care not to overbake.

Step 7

Cool until warm or room temperature. Slice into bars and devour!