A cake this good needs to feed a crowd, and this one does. It’s moist and tender, with dark swirls and curls. Find this recipe and many more delicious treats in The Old Farmer’s Almanac Everyday Baking cookbook.
Preheat the oven to 350°F. Butter a 13x9-inch baking pan and lightly dust it with flour, knocking out the excess. Set aside.
For cake: Sift the flour, baking powder, baking soda, and salt into a large bowl. Set aside. Using an electric mixer (preferably a large stand model), cream the butter on medium-high, while gradually adding the sugar. Add the eggs, one at a time, beating well after each addition. Add the vanilla and beat to blend. Add about a third of the dry ingredients to the creamed ingredients, mixing on low until evenly blended. Add half of the buttermilk, beating until blended. Repeat, ending with the remaining dry ingredients. Set aside.
Melt the chocolate chips in the top of a double boiler over not-quite-simmering water. Stir to blend. Remove from the heat and cool to lukewarm. Transfer 2 cups of the batter to a bowl and scrape the melted chocolate over it. Fold the chocolate into the batter, until evenly mixed. Set aside.
Pour half of the remaining plain batter into the prepared pan and smooth with the back of a spoon. Using a separate large spoon, spoon the chocolate batter in three evenly spaced rows, each about 1½ inches wide, lengthwise on top. Spoon the remaining plain batter evenly next to the chocolate rows. Draw the blade of a butter knife across the rows, from side to side, leaving about 2 inches between each pass of the blade and creating a swirl pattern. Do not remove the knife from the batter until you get to the end. Bake the cake on the center rack for about 40 minutes, until a tester inserted into the center comes out clean. Cool completely before frosting.
For frosting: Put 4 tablespoons of the butter into the top of a double boiler placed over not-quite-simmering water. Add the chocolate. When the chocolate has melted, stir to blend. Remove from the heat and cool the chocolate to body temperature. Using an electric mixer, blend the cream cheese and the remaining 4 tablespoons of softened butter on medium-high. Add the confectioners’ sugar, about 1 cup at a time, beating until smooth and fluffy after each addition. Beat in the vanilla. Scrape the cooled chocolate into the cream cheese mixture and beat on medium for about 30 seconds, scraping down the sides of the bowl. Add the milk, 1 tablespoon at a time, beating after each addition. The frosting should be creamy. Apply a thick layer of frosting to the cake. Serve within an hour or so, or cover loosely with foil and refrigerate. (If you refrigerate the cake, let it stand at room temperature for about 30 minutes before serving.)