Filled Cupcakes with Cocoa Frosting

Preheat the oven to 350 degrees F (175 degrees C). Line a 12-cup muffin tin with paper liners.

Sift flour, cocoa powder, baking soda, and salt for cupcakes together into a medium bowl. Combine mayonnaise, sugar, milk, and vanilla extract in a large bowl; beat with an electric mixer until smooth. Slowly add flour mixture and beat at medium speed for 2 minutes. Divide batter evenly amongst the prepared muffin cups.

Bake in the preheated oven until tops spring back when lightly pressed, 25 to 28 minutes. Cool in the tins for 5 minutes. Transfer to a wire rack and let cool completely, about 30 minutes.

While the cupcakes are baking, stir together flour and salt for the filling in a small saucepan. Gradually whisk in milk. Cook and stir over medium heat until thick and pudding-like, 5 to 10 minutes. Remove from the heat, cover, and refrigerate until cold, about 30 minutes.

Cream shortening and confectioners' sugar together until light and fluffy. Gradually mix in chilled pudding mixture and vanilla extract, beating until light and fluffy. Transfer filling to a pastry bag fitted with a small tip.

Push the pastry tip into the top of a cupcake about 1 inch deep. Squeeze filling to fill cupcake, stopping before it starts to leak outside of the hole. Repeat to fill remaining cupcakes.

Combine butter, milk, and cocoa powder for the frosting in a saucepan and bring to a boil over medium heat. Remove from the heat and transfer to a mixing bowl. Immediately add confectioners' sugar and vanilla extract; beat well. Frost cupcakes with warm frosting.