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1
Preheat the oven to 350 degrees F.
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2
Lightly butter a 9-by-13-inch baking pan, preferably a light-colored aluminum or steel pan.
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3
(Dark pans cause over-browning.)
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4
Heat a large skillet over medium heat, add the butter, and cook until the butter foams and turns amber brown, 3 to 5 minutes.
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5
Pour the butter into a large bowl and set aside until cool enough to touch.
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6
Meanwhile, sift the flour, baking powder, nutmeg and salt together into a bowl.
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7
In the bowl of a stand mixer, or using a hand mixer, combine the brown sugar with the butter and mix until combined.
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8
Add the eggs and vanilla, and mix until smooth, thick and light, 3 to 5 minutes.
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9
(You want to make sure the mixture is very light and airy, like frosting.)
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10
Stir in the sifted flour mixture and mix by hand until incorporated.
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11
Pour the batter into the prepared baking pan and spread it into an even layer, smoothing the top.
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12
Scatter the hazelnuts, pressing lightly to submerge them about halfway in the batter.
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13
Bake until the surface wrinkles evenly and turns nut brown, about 35 minutes.
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14
Let cool completely, at least 2 hours, before cutting into 2-inch bars.
