Cake:

In a mixer at medium speed, cream butter, sugar, and salt together until light and fluffy. Add eggs one at a time while the mixer is running, making sure each egg is totally mixed in before adding another. Beat mixture until light and fluffy, making sure to scrape the sides and bottom of the mixing bowl carefully.
In a separate mixing bowl, combine flour and baking powder using a wire whisk to sift and combine them. In another separate bowl, mix the milk and vanilla.  

Alternate adding wet and dry ingredients to butter mixture, starting and ending with the dry ingredients. Do not over mix, as batter should be thick and fluffy. Spoon into lined cupcake pans, filling each cup about 2/3 full. Bake at 350°F for 12 to 14 minutes or until done. 

Cream Cheese filling:

Combine shortening and cream cheese in mixer at medium speed and whip with paddle until smooth. 
Add powdered sugar and whip until fluffy; mix in root beer extract until well combined.

Frosting:

Combine all ingredients in bowl of mixer. Beat together at medium speed until light and fluffy. 

To assemble cupcakes: Use a 1/4 teaspoon measuring spoon to scoop out the center of completely cooled cupcakes from the top to about halfway down.

With a piping bag, or plastic bag with the corner snipped off, fill the hole with filling.  

Insert a dried fava bean into filling of each cupcake. Note: fava bean is for decoration only and should not be eaten. 

Using a star tip and separate piping bag, pipe frosting starting on the outside, working your way to the center in one continuous motion. Top with green, purple and yellow or gold sprinkles or decorative sugar like on a traditional King Cake.