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Quarter, core, and dice the apples.
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Toss with the lemon juice and set aside.
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Roughly chop the hazelnuts.
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Grease and flour a muffin pan. Preheat the oven to 350°F
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Whisk together the flour, baking powder, salt, and cinnamon.
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Separately whisk the egg, sugar, oil, vanilla, and buttermilk until combined.
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Add the flour mixture. Fold in the diced apples and half of the hazelnuts.
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Transfer the batter into the muffin tins. Arrange the remaining hazelnuts on top.
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Sprinkle with the cinnamon sugar.
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Bake 25 minutes until golden brown or until a knife or tester inserted into the middle comes out clean.
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Let cool, then de-pan and serve warm.