There are a number of ways one can make filled cupcakes. Many involved coring out the center of the cupcake, removing a good portion of the cake, so that it can be filled. For regular size cupcakes I find that this method ends up getting you a cupcake that is mostly frosting and filling with a hint of cake.
Instead I like to use a piping bag and tip #230 to fill my cupcakes. This tip is long with a round opening so that you can insert it deep into the cupcake. Most methods that use tip #230 tell you to just fill in the center of the cupcake. But doing just one squirt in the center doesn’t seem to get enough filling. Instead I like to fill using at least 4 squirts. Be sure to use a coupler on your pastry bag when using this tip. It keeps it in place when you push it into the cupcake.
First off I start out with my cupcakes cold. They are much less crumbly when they are cold, which makes them easy to handle, but that also makes them a little harder to fill. That’s why my first step is to use a modeling stick or a skinny dowel/skewer to poke a few pilot holes into the cupcake. I like to use 4 squirts of filling evenly spaced since that is approximately how many bites it takes to eat a cupcake. That way you get a little bit of filling in each bite.
Once I’ve poked all my pilot holes I use tip #230 to squirt a little filling into each whole. I tend to let the filing overflow the holes a bit since I cover it with frosting using a tip #1M swirl, which makes it easy to cover. If you will be frosting with a spatula you shouldn’t let the filing overflow much since it will make it harder to frost cleanly.