In a small saucepan over medium heat, melt the butter. Continue cooking until golden brown and nutty-smelling, about 5-8 minutes. Let cool for 15-20 minutes
Don’t strain the butter, keep and include the dark bits, as they’re packed with flavor.
Preheat the oven: Once the butter has cooled, Preheat the oven to 350°F / 180°C (160°C fan-forced)
Grease and line an 11.5 x 8-inch (29 x 20 cm) baking pan with parchment paper (baking paper) extending over the edges. (See note 3 for alternative pan sizes.)
Dry ingredients: In a large bowl, whisk together the almond flour, powdered sugar, all-purpose flour, and salt.
Egg whites: In a separate bowl, whisk the egg whites until frothy (no need to whip to peaks).
Combine: Add the frothy egg whites to the cooled butter and stir to combine. Then add this mixture to the dry ingredients and fold gently until combined.
Pour the batter into the prepared pan and spread it evenly using an offset spatula or by gently tilting the pan to level it out
Optional: Dollop small spoonfuls of jam (about 1 teaspoon each) across the surface. Use a toothpick or knife to swirl the jam into the batter.
Bake for 15–18 minutes or until golden brown, and a toothpick inserted into the center comes out clean.
If you prefer a deeper golden color, you can bake for an additional 3–5 minutes, checking every 1–2 minutes after the initial baking time to ensure they don't over-bake
Cool and Slice: Cool in the pan for 5 minutes before lifting with the parchment paper and transferring to a wire rack. Slice into squares or rectangles for serving.