Buttery and Soft Chocolate Cake for a Crowd

For Cake: Preheat the oven to 350 degrees F. Lightly grease the inside of a 13-by-9-by-2-inch baking pan with shortening and dust with flour.

Combine the chocolate and water in a small, heavy-bottomed saucepan over low heat, stirring as the chocolate melts and until the mixture is smooth. Set aside; by the time you need to use it, it should be slightly warm and have the texture of thick pudding.

Sift the flour, baking powder, baking soda and salt onto a sheet of wax paper.

In the large bowl of a stand mixer on medium speed, beat the butter for 2 to 3 minutes, until creamy and light. Add the brown sugar in 2 additions, beating for 1 minute after each portion is added. Add the sugar and beat for 1 minute.

Add the eggs, one at a time, beating for 45 seconds after each addition, then add the egg yolk. Reduce to low speed and add the chocolate mixture and vanilla extract. (The mixture may look slightly curdled at this point but will recover once the sifted mixture and sour cream are added to complete the batter.)

On low speed, alternately add the sifted mixture in 3 additions with the sour cream in 2 additions, beginning and ending with the sifted mixture. Scrape down the sides of the mixing bowl as needed. The batter will be soft, light and very creamy.

Spoon the batter into the prepared pan and spread evenly, smoothing the top with a spatula. Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean and the cake has pulled away slightly from the sides of the pan.

Transfer the pan to a wire rack to cool, then spread the frosting over the top of the cake. Let the cake stand for 1 hour before cutting into squares for serving.

For Chocolate Frosting: In the bowl of a stand mixer on medium speed, beat the butter for 1 minute, until creamy. Add the melted chocolate, salt, vanilla extract, half of the milk and 1 1/2 cups of the confectioners' sugar. Add the remaining the milk and beat for 1 minute.

Add half of the remaining confectioners' sugar and beat for 1 minute or until smooth. Reduce the speed to low and add the remaining confectioners' sugar, then increase speed to medium-high and beat for about 3 minutes, until the mixture is very creamy and somewhat lightened in texture. Scrape down the sides of the bowl as needed.

Adjust the texture of the frosting to soft spreading consistency, as needed, by adding teaspoons of milk or tablespoons of confectioners' sugar. Cover until ready to use.