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1
For the cake: Preheat the oven to 350 degrees F. Butter an 8-by-2-inch round cake pan and line it with parchment.
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2
In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder and salt.
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3
In a large bowl, whisk together the sugar, melted butter and eggs.
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4
Alternate whisking the flour mixture and the buttermilk and coffee into the sugar and egg mixture, starting and ending with the flour mixture.
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5
Transfer the batter to the prepared pan.
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6
Bake until a toothpick inserted into the center comes out with moist crumbs attached, about 40 minutes.
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7
Transfer to a rack to cool for 20 minutes, then flip the cake out onto the rack to cool completely, about 1 hour.
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8
For the custard: Put the chocolate and butter in a medium bowl and set a fine-mesh sieve over it.
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9
In another medium bowl, whisk the sugar, cornstarch, salt and egg yolks together until smooth.
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10
Bring the milk and the cocoa powder to a simmer in a small pot.
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11
While whisking, add about 1/4 cup of the hot milk mixture to the yolk mixture.
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12
Repeat this process until the two are completely combined.
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13
Return the mixture to the pot and heat over medium while stirring constantly.
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14
Bring the custard to a boil and cook, whisking constantly, for 1 to 2 minutes.
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15
Immediately pour the custard through the sieve into the bowl with the chocolate.
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16
After the butter and chocolate have melted, whisk until smooth.
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17
Cover with a piece of plastic wrap, making sure the plastic touches the surface of the custard, and chill until completely cold, about 1 hour.
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18
For the glaze: Put the chocolate in a medium heatproof bowl.
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19
Bring the cream to a boil in a small saucepan and pour it over the chocolate.
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20
Let the mixture stand until smooth and then whisk it to combine.
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21
Whisk in the salt.
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22
Chill the glaze, stirring occasionally, until it is thick enough to spread over the top of the cake, 20 to 30 minutes.
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23
To assemble the cake, cut it in half crosswise.
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24
Spread the chocolate custard evenly over the bottom half, and replace the top half.
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25
Spread the glaze evenly over the top.
