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1
For the filling:
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2
Heat the milk in a medium saucepan over medium-high heat, just until small bubbles begin to form around the edge of the pan.
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3
Meanwhile, whisk together the sugar, flour, and salt in a small bowl. Set aside.
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4
Whisk the egg yolks in a medium bowl until well blended. Slowly whisk the flour mixture into the eggs, until very thick. Slowly add the warm milk to the egg mixture, whisking constantly until incorporated.
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5
Return the mixture to the saucepan and place over medium heat. Whisk for about 3 minutes, until the mixture thickens and comes to a boil. Remove from the heat and whisk in the hazelnut paste and vanilla extract.
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6
Pour the custard mixture into a bowl. Cover the custard with plastic wrap, pressing the wrap directly onto the surface of the cream. Chill in the refrigerator for at least 4 hours, or overnight.
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7
For the cupcakes:
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8
Preheat the oven to 350u00b0F. Line two standard-size 12-count muffin tins with paper liners (recipe makes roughly 20 cupcakes so you may not need to line all of both pans).
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9
Melt the chocolate in the top of a double boiler with simmering water in the bottom, stirring until completely smooth. Remove from the heat and set aside to cool.
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Combine the sugar, flour, baking soda, and salt in the bowl of an electric mixer and blend for a few seconds to combine.
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11
Whisk together the coffee, sour cream, and vegetable oil in a small bowl. With the mixer on low speed, slowly add the coffee mixture and mix until well combined. Add the eggs, one at a time, blending well after each addition. Stir in the melted chocolate.
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12
Using an ice cream scoop, scoop the batter into the muffin cups. Bake for 18 to 20 minutes or until a cake tester inserted in the center of a cupcake comes out clean. Remove from the oven and let them cool in the pan for 10 minutes. Transfer the cupcakes to a wire rack and let cool completely.
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13
For the ganache:
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14
Melt the chocolate and heavy cream in the top of a double boiler with simmering water in the bottom, stirring until completely smooth. Remove from the heat and let cool slightly.
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15
For the assembly:
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16
Spoon the hazelnut pastry cream filling into a pastry bag fitted with a large round tip. Gently insert the tip of the pastry bag into the top of one cupcake and fill with some pastry cream. Carefully dip the top of the cupcake into the ganache to coat. Place one toasted hazelnut on top of the cupcake. Set it on a sheet of parchment paper. Repeat with the remaining cupcakes, filling and ganache. Let cupcakes sit at room temperature for about 1 hour, until the ganache hardens.
