To make the buttercream, start with the pudding base: Place the chocolate in the clean bowl of the stand mixer fitted with the paddle attachment. In a small saucepan, combine the milk and ¼ cup/50 grams of the sugar. Bring to a simmer over medium, stirring occasionally to dissolve the sugar, then remove from the heat and set aside. In a medium bowl, add the cocoa powder, flour, salt and remaining ¼ cup/50 grams sugar, and whisk until combined. Add the yolks to the bowl and whisk slowly to combine (it will seem very thick at first, but will loosen up), then whisk more vigorously until the mixture is pale, thick and smooth. Whisking the yolk mixture constantly, slowly stream in about half of the warm milk mixture, then whisk the contents of the bowl back into the saucepan. Set the saucepan over medium-low heat and cook, whisking constantly and scraping against the sides, until thick and bubbling, 1 to 3 minutes. Immediately scrape the pudding over the chocolate in the stand mixer bowl.