Adapted from Michael Recchiuti and Fran Gage's Chocolate Obsession.
1/4 cup (4 ounces) heavy whipping cream
1/4 cup plus 3 tablespoons (4 1/2 ounces by weight) invert sugar (stir before measuring)
1 Tahitian vanilla bean, split horizontally
12 ounces 61-70% chocolate, finely chopped
3 tablespoons (1 1/2 ounces) unsalted butter with 82% butterfat, like Plugrá, very soft (75 degrees)
Unsweetened cocoa powder, or another garnish, for rolling
Stir the cream and invert sugar together in a medium saucepan. Scrape the vanilla seeds from the bean into the pan and then add the bean. Bring to a boil over medium heat, remove from the heat, and cover the top of the pan with plastic wrap. When the cream has cooled to room temperature, transfer it to a bowl, cover, and refrigerate overnight.
Line the bottom and sides of an 8-inch square baking pan with plastic wrap. Put the chocolate in a medium stainless steel bowl and set the bowl over a pot of simmering water. Heat, stirring occasionally, until the chocolate melts and registers 115 degrees on an instant-read thermometer. Lift the bowl from the pot.
When the chocolate is almost 115 degrees, remove the cream from the refrigerator. Strain it through a fine-mesh sieve into a small saucepan and heat it to 115 degrees, stirring occasionally.
Pour the chocolate and cream into a 1-quart clear vessel. Blend with an immersion blender using a stirring motion and making sure you reach the bottom of the vessel. The ganache will thicken, become slightly less shiny, and develop a pudding-like consistency. Add the butter and incorporate it with the immersion blender.
Pour the ganache into the lined pan. Spread it as evenly as possible with a small offset spatula. Allow the ganache to cool at room temperature until it has set, 2–4 hours. Cover the pan with plastic wrap and refrigerate until you are ready to roll truffles.