First, start by combining the eggs, white granulated sugar, oil and vanilla extract in a stand mixer or in a bowl if using a hand mixer. Mix on low speed until well combined.
Next, in a separate bowl, combine the flour, unsweetened cocoa powder, baking powder and salt. Whisk together to break up any lumps.
Add half of the dry ingredients to the wet ingredients while mixing on low speed. Add the milk, then add the rest of the flour and cocoa powder mixture to the cupcake batter. Mix on low until well incorporated. Do not over mix.
Preheat the air fryer to 350 degrees F for 5 minutes.
Grease your cupcake liners and insert them into a cupcake or muffin pan that will fit your air fryer.
Cut the fun sized Twix bars in half.
Next, add 2 tbsp of the cupcake batter to the cupcake liners.
Place half of a fun sized Twix bar on top of the cupcake batter.
Add 2 more tbsp of cupcake batter on top of the Twix bar piece. The cupcake liners should be about 3/4 full.
Make sure to use a small spatula to smooth out the top if needed. You also want to make sure the Twix piece is completely covered by the cupcake batter.
Place the cupcakes inside the air fryer and air fry for 11 minutes at 350 degrees F.
Gently remove them from the air fryer and let them cool completely before frosting them with the caramel buttercream frosting.