This cake has a rich chocolate flavor and moist tender crumb. Unsweetened chocolate gives a slightly deeper chocolate flavor but you can also use semisweet chocolate chips. Please read About High-Altitude Cake Baking and About Butter Cakes. This recipe is designed for baking at 5,000 feet.

If baking at 7,000 feet, replace the cake flour with sifted all-purpose flour (240g) and reduce the baking powder by ½ teaspoon. Replace the regular milk with buttermilk and increase it by 1 tablespoon (15g). Reduce the sugar beaten into the whites to 2 tablespoons (25g). Bake layers or a sheet cake for 35 to 40 minutes, cupcakes for 15 to 20 minutes.

If baking at 10,000 feet, replace the cake flour with sifted all-purpose flour and increase by 1 tablespoon (250g total), and reduce the baking powder by ¾ teaspoon. If using unsweetened chocolate, reduce the sugar beaten with the butter by ½ cup (100g); if using semisweet chocolate, reduce it by ¾ cup (150g). Replace the milk with buttermilk and increase it by 2 tablespoons (30g). Bake layers about 33 minutes, a sheet cake about 42 minutes, or cupcakes 25 minutes.

Have all ingredients at room temperature, about 70°F. Preheat the oven to 350°F. Thoroughly grease and flour two 9-inch round cake pans, one 13 × 9-inch baking pan, or two 12-cup muffin tins. Line round cake pans with parchment paper and muffin tins with paper liners.

Melt, stirring constantly, in a double boiler or heatproof bowl over barely simmering water:

Whisk together thoroughly in a medium bowl:

Beat in a large bowl, or in a stand mixer with the whisk attachment, until foamy:

Gradually add and beat on high speed until soft, slightly droopy peaks form:

Set aside. Beat in another large bowl (or transfer the egg whites to another bowl and reuse the stand mixer bowl with the paddle attachment) until light and fluffy:

Scrape down the bowl and beat a few seconds longer. Beat in, blending well:

Beat in the melted chocolate. Scrape down the bowl. On low speed, add the flour mixture in 3 parts, alternating with:

in 2 parts, scraping down the bowl and beating well after each addition. Gently fold in the whites by hand in 3 additions until the batter no longer looks streaky. Divide the batter between the round pans, scrape into the baking pan, or, for cupcakes, fill cups almost three-quarters full. Bake until a cake tester inserted into the center comes out clean, 35 to 40 minutes for layers or a sheet pan, 15 to 20 minutes for cupcakes.

Let cool in the pan(s) on a wire rack for 10 to 15 minutes. Then unmold and cool completely. (The sheet cake can be cooled in and served from the pan.)