Whereas the truffles bake, clear the blending bowl and make the buttercream: Add the butter to the bowl and blend on medium-high till clean, stopping the mixer often to scrape the underside and sides of the bowl. Add the confectioners’ sugar, cocoa powder, bitter cream, milk, vanilla extract and salt. Combine on low till the sugar and cocoa are moistened, then flip the velocity as much as medium-high and whip till clean and fluffy, about 5 minutes, including a bit extra milk (1 teaspoon at a time) if essential to make a clean and lightweight buttercream. Scrape the bowl often to make sure that the buttercream is evenly blended. Style and add a bit extra salt if desired. (The buttercream may be stored at room temperature whereas the cake finishes baking and cooling.)
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