Back when I first started investigating how to make an eggless chocolate mousse based on homemade condensed milk, I did a few trials that included a little butter whipped into the inky Dutch chocolate "pudding" base.
Many recipes for chocolate mousse melt dark chocolate with butter to make a ganache, so the idea was perfectly natural in terms of flavor and richness, if not technique. By whipping cool butter into the base rather than melting it with dark chocolate, I'd hoped to make an extra-fluffy sort of mousse. What I got instead was a chocolate buttercream.
What sets this mousse-like frosting apart is that it's eggless, making it a friendlier style for folks with allergies, as well as anyone not armed with the sheer volume of eggs required for a Swiss or French buttercream. And, unlike other dairy-forward frostings, such as German buttercream, this one contains no cornstarch, a problematic ingredient for allergic children.
Because this frosting's based on dairy, it has an especially creamy texture and flavor, one that mellows the intensity of Dutch cocoa into something wonderfully mild (think hot cocoa or chocolate milk). Since it doesn't actually involve any dark or milk chocolate, only a small amount of cocoa powder, it's an extremely affordable chocolate frosting that still tastes super luxe, a major boon to those of us who can't help but think of recipes in terms of food cost.
Then I bring the mixture to a simmer and cook, stirring constantly, until it reduces to an inky-black sort of pudding.
I scrape it into a large baking dish to speed the cooling process and refrigerate until it's cool and thick, but not cold—about 70°F (21°C). This step can be done up to a week in advance, in which case, after refrigeration, the pudding will need to stand at room temperature until warmed to about 70°F.
Either way, I then transfer the cool pudding to the bowl of a stand mixer fitted with a paddle attachment, along with some butter softened to the same temperature, and beat them together until creamy and light.
As with any frosting, the working temperature of this recipe will affect its consistency and yield. When too cold, it will be heavy, dense, and a little greasy, with a low overall yield. So if it seems too stiff to spread, or if it looks slightly grainy or curdled, pop it over a simmering water bath for a few seconds to knock off the chill, then re-whip.
The result will be about five cups of a deliciously thick and creamy milk chocolate frosting that's spreadably smooth, with a sweetness that's balanced by its rich chocolate flavor.