WHEN Marie Antoinette sneered "Let them eat cake!" the point was that she wasn't promising them frosting. Cake is but an excuse. And as anyone with candor and taste (meaning anyone under 18) will tell you, the reason we have cake is to serve the frosting on something.

Thus whether you unwrap, empty-and-mix or start your cake from scratch, the crucial decision is what's on top. It gives no pause when a cook makes the frosting from start to finish for a cake-mix cake, but we never expect to find a packaged frosting mix used for a home-recipe cake.

So you provide the cake, we've got the frosting. We've sought the best. We culled through our files and books, then tested 10 different frostings--chocolate, naturally--and tasted them blind. We confess the tasting wasn't truly scientific: we had no beaters to lick. So we had to test the frostings just on cake, or, somewhat more authentically, from our fingers. We also recognize that there are strongly divergent opinions that decree only fudge or only buttercream or only bittersweet glaze as the true chocolate frosting.

This is the one that delighted most of the people most of the time, and for good reason. It is smooth and silky from the butter and large number of egg yolks. It is soft and fluffy, yet rich; its chocolatiness is light, but not lost. The cooking method is its secret, allowing the eggs to warm slowly and lend body and airiness like a chocolate zabaglione. It spreads smoothly and swirls easily into peaks. It is the American dream of a chocolate frosting. MAIDA HEATTER'S CHOCOLATE BUTTERCREAM (Enough for 2 9-inch layers) 6 ounces semisweet chocolate 2 tablespoons heavy cream 2 sticks plus 1 tablespoon unsalted butter 7 egg yolks 1 cup strained confectioners' sugar Pinch salt 1 teaspoon vanilla

Break up or coarsely chop the chocolate and place it in the top of a small double boiler over hot water on low heat. Add the heavy cream and 1 tablespoon of the butter (reserve the remaining 2 sticks at room temperature). Stir until smooth.

Meanwhile, in the small bowl of an electric mixer, beat the egg yolks at high speed for a few minutes until pale and thick. On low speed gradually add the warm chocolate mixture, scraping the bowl constantly with a rubber spatula.

Transfer the mixture to the top of the double boiler (the one the chocolate was melted in) over hot water on low heat and cook for 5 minutes, scraping the bottom and sides constantly with a rubber spatula. The mixture must never get really hot.

Now transfer the chocolate mixture to a mixing bowl. Place some ice and water in a larger mixing bowl. Then place the bowl of chocolate into the bowl of ice water and stir gently until the chocolate cools to tepid.

Meanwhile, in the large bowl of an electric mixer, beat the reserved 2 sticks of butter with the confectioners' sugar, salt and vanilla until soft and smooth. Then add the chocolate mixture and continue to beat for just a minute or two until smooth, creamy and gorgeous.

Just before you are ready to frost the cake, beat the reserved buttercream well with the mixer.