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1
Preheat oven to 350 degrees F.
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2
Butter a 10-inch round cake pan and line the bottom with waxed or parchment paper.
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3
Flour the sides of the pan with matzo cake meal.
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4
In the top of a double boiler set over barely simmering water, melt the butter and chocolate together, stirring frequently.
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5
Set aside to cool slightly.
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6
In a mixer fitted with a whisk attachment (or using a hand mixer), whip the eggs, egg yolks, and sugar until very light and fluffy.
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7
Fold in the hot chocolate mixture.
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8
In a small bowl, mix the matzo meal and ground walnuts.
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9
Fold into the chocolate-egg mixture.
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10
Pour into the prepared pan and bake until firm and dry on the top and a toothpick inserted into the center comes out almost clean (a few crumbs are OK), about 35 to 40 minutes.
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11
Let cool 20 minutes in the pan, then turn out onto a rack and let cool to room temperature.
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12
Transfer to a serving plate and refrigerate 1 hour.
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13
Meanwhile, make the frosting: In the top of a double boiler set over barely simmering water, whisk the egg whites and sugar together until the mixture is very hot and the sugar has dissolved somewhat.
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14
Remove from the heat and whip the mixture in a mixer (or using a hand mixer) until cooled, smooth, and fluffy.
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15
Add the butter and whip until smooth; the mixture might look broken at this point, but keep whipping and it will smooth out.
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16
Drizzle in the melted chocolate and coffee, and whip until smooth and fluffy.
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17
Using a flexible spatula, frost the chilled cake on the top and sides.
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18
Serve immediately or refrigerate up to 24 hours.
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19
Remove from the refrigerator 1 hour before serving.
