Melt 8 ounces dark chocolate in a double boiler (or, make a homemade version by placing a heatproof bowl over a sauce pan filled with 2 inches of water over medium heat, making sure the bottom of the bowl does not touch the water). Use a rubber spatula to stir until smooth, then remove from heat to cool slightly.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, combine 1 can unsweetened chestnut purée, 1/2 cup unsalted butter, 1/2 cup granulated sugar, and 1/2 teaspoon kosher salt. Beat on medium speed until well combined and smooth, 2 to 3 minutes. With the mixer on low speed, slowly pour in the melted chocolate and continue beating until combined. For the Chocolate Ganache Frosting:
Combine 10 ounces bittersweet chocolate and 2/3 cup confectioner's sugar in a medium, heatproof bowl.
In a small saucepan over medium, bring 3/4 cup plus 1 tablespoon heavy cream to a gentle simmer, whisking occasionally. Pour the hot cream into the medium, heatproof bowl containing the chocolate and confectioner's sugar mixture (from the 1st step). Stir the mixture with a heatproof, rubber spatula until the sugar is dissolved.
Once the sugar has dissolved, nestle the medium, heatproof bowl over a medium saucepan of simmer water to make a double boiler — be sure that the water in the saucepan doesn't touch the bottom of your heatproof bowl. Gently cook, stirring with a rubber spatula, until all the chocolate is melted and the mixture is smooth. Remove the bowl from the heat.
In a separate, smaller heatproof bowl, whisk together 2 large egg yolks. Pour about 1/2 cup of some of the melted chocolate mixture into the yolks while whisking, to temper them. Pour the tempered mixture back into the larger, heatproof bowl containing the rest of the original mixture and whisk to combine. Add 3 tablespoons unsalted butter and stir until smooth.
Allow the ganache to cool for about 10 - 15 minutes, before using immediately.