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1
make the cake Preheat the oven to 375.
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2
Spray an 8-inch square baking pan.
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3
Line the bottom and 2 sides with parchment paper, allowing the paper to extend 1 inch beyond the pan.
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4
Spray the paper.
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5
make the cake In a pie plate, toast the hazelnuts in the oven for 12 minutes, until the skins blister.
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6
Spread the nuts on a kitchen towel and let cool completely.
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7
Rub the nuts together to rub off the skins.
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8
Transfer the nuts to a food processor.
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9
Add the flour and salt and pulse until the nuts are finely ground.
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10
make the cake In a microwave-safe glass bowl, heat the chocolate at 20-second intervals, stirring a few times, until melted.
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11
Let cool to warm.
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12
In a bowl, beat the butter with the sugar at medium-high speed until fluffy, 2 minutes.
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13
Add the chocolate, egg yolks and vanilla and beat until combined.
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14
Beat in the dry ingredients just until incorporated.
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15
make the cake In another bowl, using clean beaters, whip the egg whites at high speed until soft peaks form.
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16
Fold the whites into the batter until no streaks remain.
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17
Scrape the batter into the prepared pan and smooth the surface.
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18
Bake in the center of the oven for 30 minutes, until a skewer inserted in the center comes out with a few moist crumbs attached.
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19
Let the cake cool in the pan for 30 minutes, then freeze until chilled, 1 hour.
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20
make the cake Line a baking sheet with parchment paper.
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21
Invert the cake onto a cutting board and remove the paper.
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22
Cut the cake into 5 equal strips, then cut each strip crosswise into 5 squares, wiping the knife between cuts.
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23
Transfer the squares to the prepared baking sheet.
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24
make the ganache In a microwave, heat the cream and butter until very hot.
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25
Whisk in the espresso powder.
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26
Add the chocolate and let stand until melted; whisk until smooth.
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27
make the ganache Using a small offset spatula, spread the ganache on the top and sides of each square.
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28
Sprinkle the squares with chopped hazelnuts and refrigerate until the ganache is firm, about 30 minutes.