Preheat oven to 350 degrees F (175 degrees C). Grease two 12-cup muffin tins.
Sift flour, 2 cups white sugar, cocoa powder, baking powder, and salt together in a large bowl. Beat vegetable oil, buttermilk, eggs, and 1 teaspoon vanilla extract into the flour mixture using an electric hand mixer on medium until smooth, about 3 minutes.
Stir baking soda and hot water together in a small bowl; mix into batter. Pour batter into muffin cups to 2/3 full.
Bake in the preheated oven until a toothpick inserted in the center of a cupcake comes out clean, about 20 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.
Beat shortening, 1/2 cup white sugar, milk, 1 tablespoon water, 1 teaspoon vanilla extract, and salt together in a bowl using an electric hand mixer until smooth, 5 to 7 minutes. Add confectioners' sugar; beat until fully incorporated, 3 to 5 minutes.
Insert frosting into cupcakes using a pastry tube fitted with a rosette tip. Frost the tops of cupcakes with remaining frosting.