Yellow Cake with Fudge Icing

Heat oven to 350°. Grease two 9" round cake pans with shortening; dust with flour. Shake out excess flour and line pan bottoms with parchment paper cut to fit; set aside. In bowl of a stand mixer fitted with paddle, beat together 1 cup shortening and 2 cups sugar on medium-high speed until fluffy, 1–2 minutes. Add eggs one at a time, beating well after each, until incorporated; add 1 tsp. vanilla. Reduce mixer speed to low; alternately add flour and buttermilk in 3 batches. Mix until smooth. Divide batter between pans. Bake until a toothpick inserted in middle of cakes comes out clean, 35–40 minutes. Set on a rack to cool completely. Remove cakes from pans; set aside.

Whisk together remaining sugar and the cocoa, and then the milk, in a 2-qt. saucepan. Bring to a boil over medium heat while stirring. Attach a candy thermometer to side of pan; cook, without stirring, until thermometer reads 242°. Remove pan from heat. Stir together remaining vanilla and the baking soda; add to chocolate mixture, along with the butter, and stir until butter is melted. Let icing sit until it cools to 120°. Stir mixture with a wooden spoon (or in bowl of a stand mixer fitted with paddle on medium speed) until icing thickens considerably but is still loose enough to be spread. Working quickly, place one cake layer, top side up, on a cake stand and use a butter knife to spread one-third of icing over top. Place second cake layer, top side down, on top of first, and spread remaining icing over top and side of cake. Chill until icing sets, about 2 hours.