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1
Place the chocolate in a medium bowl and set aside.
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2
Combine the milk and all but 1-tablespoon of the sugar in a small saucepan.
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3
Heat over medium heat, stirring constantly until the sugar is completely dissolved.
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4
Stop stirring and cook until the milk begins to bubble around the edge of the pan.
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5
Remove the pan from the heat.
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6
Whisk together the egg yolk and the remaining 1-tablespoon sugar in a small bowl.
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7
Pour half of the hot milk over the yolk mixture, whisking constantly to combine.
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8
Return the mixture to the saucepan and cook, whisking constantly, until the mixture thickens slightly and reaches 185 degrees F on an instant-read thermometer.
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9
Immediately pour the mixture over the chocolate.
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10
Whisk until the chocolate is melted.
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11
Add the butter 1/2-tablespoon at a time, whisking until the butter is melted and the mixture is smooth.
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12
Whisk in the vanilla.
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13
Scrape the batter into a 6-inch round cake pan and smooth the top with a rubber spatula.
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14
Cover the pan with plastic wrap and place it in the freezer for at least 6 hours, or overnight.
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15
To unmold the cake, carefully immerse the pan in hot water for a few seconds.
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16
Wipe the bottom and sides of the pan dry and invert the cake onto a serving platter.
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17
Decorate the top of the cake by running a hot dull knife across its surface in a wave pattern (dip the knife in hot water and wipe completely dry before using it).
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18
Lightly dust the top of the cake with sifted cocoa powder.
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19
Allow the cake to stand at room temperature for 45 minutes before serving.
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20
Store leftovers in the refrigerator.
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21
Note: It is important that the butter be at room temperature, or it will not blend completely into the chocolate mixture and there will be a small lumps of butter in the cake.
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22
You can make this 2 or even 3 days in advance and freeze it.
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23
When you need it, unmold it and let it stand at room temperature for 45 minutes so its not freezer cold.
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24
Or unmold it and then store in the freezer, whole or cut into portions, for up to 2 months.
