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1
Pulse hazelnuts in food processor until no large pieces remain. This should take about 8 to 10 long pulses.
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2
Add the brown sugar, flour, salt, nutmeg, and cinnamon. Pulse several times to combine.
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3
Add the butter, and pulse until everything is incorporated and the mixture looks damp and clumpy, about 8 to 10 pulses. Set aside until ready to sprinkle on the buckle.
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4
Preheat oven to 350u00b0 F with a rack in the lower middle position. Butter and flour a 2-inch deep, 9-inch square, 10-inch round, or 11- by 7-inch rectangular pan. Set aside.
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5
Combine flour, baking powder, and salt in a large bowl. Whisk until well mixed, about 15 seconds.
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6
Place butter in a glass bowl or 4-cup glass measure. Microwave for 25 to 35 seconds, until about half melted. Add the sugar, egg, and vanilla to the bowl, then whisk until combined; the mixture should look slightly fluffy. Gradually beat in the buttermilk (if the buttermilk is too cold, the mixture will look a bit curdled, but don't worry. It'll still make a delicious buckle).
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7
Add the wet ingredients to the dry ingredients, and stir just until the batter is smooth. Gently fold in the berries (some of them will break and discolor the batter, but it will come out fine in the end).
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8
Transfer the batter to the prepared pan, spreading it evenly. Sprinkle with the streusel topping in an even layer. Bake for 40 to 55 minutes (depending on your pan and oven). The top will be golden brown, and a toothpick inserted near the center won't have any batter clinging to it (though it may have berry juice or streusel on it).
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9
Let cool on a wire rack for at least 20 minutes before serving. Serve warm or at room temperature. No one will complain if you add a dollop of whipped cream or a scoop of ice cream, but it's also good enough to eat plain.
