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1
Cupcakes:.
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Heat an oven to 350F Line 8 wells of a muffin pan with cupcake papers.
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Put the hazelnuts and sugar into the bowl of a mini chopper and pulse until the nuts are ground you want some texture so do not grind them too finely.
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Evenly distribute the ground nuts between the lined wells of the pan and set the pan aside.
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In a small bowl, sift together the flour, baking powder, and salt.
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Set aside.
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In another bowl, whisk together the yogurt, milk, and vanilla.
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Set aside.
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In the bowl of a mixer fitted with the paddle attachment, cream the butter and light brown sugar on medium speed until light and fluffy, 2-3 minutes.
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Scrape down the sides of the bowl, add the egg, and mix until combined.
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Add half of the flour mixture, all of the yogurt mixture, and then the remaining flour, mixing on medium speed after each addition until just combined.
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Distribute the batter between the cups.
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Bake until lightly golden around the edges and a toothpick comes out clean, 16-18 minutes.
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Frosting:.
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Slice 4 berries in half lengthwise and set them aside.
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Puree the remaining berries along with the sugar.
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Strain the blackberry puree through a fine-mesh sieve into a small saucepan.
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Gently simmer the puree over medium-low heat until it is thick and reduced to about 2 tablespoons; this will take 10-12 minutes.
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Remove the saucepan from the heat and set it aside to cool.
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Put the sugar and egg whites in the bowl of a standing mixer.
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Bring 2 inches of water to a simmer in a medium saucepan and set the mixer bowl on top of the pan.
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Whisk the egg white mixture until it is 130 F the mixture will turn thick and white, and this should take about 3 minutes.
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Transfer the bowl to a standing mixer fitted with a whisk attachment.
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Whip the egg white mixture on high speed until it is cool, thick, and glossy, about 7 minutes.
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Reduce the speed to medium-high and add the butter a few pieces at a time, making sure they are fully incorporated before the next addition.
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After all the butter has been added, continue beating the frosting until it comes together this will take a minute or two.
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Add the reduced blackberry puree (or jam) and mix to combine.
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Frost the cupcakes and top each with a berry half.
