For cake, grease an 8x8x2- or 9x9x2-inch baking pan. Set aside.
In a small bowl, stir together flour, cocoa powder, baking powder, and dash salt. In a medium mixing bowl, beat the 2 egg yolks and the 2/3 cup sugar with an electric mixer on high speed for 3 to 4 minutes or until mixture is thick and light colored. Reduce speed to medium and gradually beat in the boiling water and vanilla. Continue beating on high speed about 5 minutes or until the mixture is thick enough to hold a ribbon on the surface when you lift the beaters. By hand, fold in dry ingredients.
Thoroughly wash the beaters. In another medium bowl, beat egg whites until soft peaks form (peaks curl). Fold about half of the egg whites into cake batter. Fold in remaining egg whites. Spread the batter into prepared pan.
Bake in a 350 degree F oven for 20 to 25 minutes or until top springs back when lightly touched. Cool about 10 minutes; remove from pan. Let cake cool completely on wire rack. Using a serrated knife, cut cake into 1-inch cubes.
For zabaglione cream, in the top of a double boiler, beat the 4 egg yolks with the Marsala, the 1/4 cup sugar, and dash salt. Place over boiling water (upper pan shouldn't touch the water). Beat with an electric mixer on medium speed until mixture triples in volume and temperature of mixture reaches 145 degree F for 3-1/2 minutes. Place in a larger bowl of ice water and continue beating until the zabaglione has cooled.
In another bowl, beat the whipping cream until soft peaks form (tips curl). By hand, fold together the zabaglione and the whipped cream.
Pour about 1/4 of the zabaglione cream into a 1-1/2-quart clear glass serving bowl or trifle bowl. Top with 1/3 of the cake cubes. Sprinkle the cake generously with 2 to 3 tablespoons of the espresso. Continue layering the remaining zabaglione cream and cake cubes sprinkled with the espresso two more times, ending with a total of 3 layers of cake and 4 layers of cream.
Using a vegetable peeler, shave semisweet chocolate curls evenly over the trifle. Cover with plastic wrap and refrigerate for at least 4 hours. Makes 5 or 6 servings.
Prepare trifle up to 24 hours ahead. Cover and refrigerate.