Preheat the oven to 350 degrees. Grease a 9x13-inch baking pan with butter, line the bottom with parchment paper, then grease and flour the pan.
Place the hazelnuts on a sheet pan and roast for 10 minutes, until the nuts are well-toasted and the skins have begun to split. Cool slightly, then roll the hazelnuts between your hands to remove the skins (it’s fine if they don’t all come off.) Discard the skins, then roughly chop the hazelnuts and set them aside.
In a large bowl, combine the butter, brown sugar, eggs, egg yolks, vanilla, and espresso powder, and whisk until smooth.
In a small bowl, combine the flour, baking powder, and kosher salt. Gradually mix the dry ingredients into the wet ingredients and stir until just combined, making sure to scrape the bottom and sides of the bowl. Add both chocolates and the hazelnuts and mix until just combined.
Transfer the batter to the prepared pan, smooth the top with a spatula, and sprinkle with flaky sea salt. Bake for 25 to 27 minutes, or until the edges of the blondies are golden brown and the center is just barely set. (Don't over-bake!)
Cool completely in the pan before cutting the blondies into squares. Store in a sealed container at room temperature or in the refrigerator for up to 3 days.Copyright 2020, Lidey Heuck, All Rights Reserved