Preheat the oven to 350 °F. Line a standard cupcake pan with cupcake liners.
Sift together the flour and salt in a bowl and set aside.
Place the butter in the bowl of a stand mixer (or use a hand-held electric mixer). Beat on medium speed until fluffy. Add the sugar and beat on medium speed until well incorporated.
Add the egg and beat on low. Add the red food coloring, vanilla extract, and cocoa powder to the bowl. Mix on low until blended.
Add half of the flour mixture to the bowl and slowly pour in half the milk, beating until well incorporated. Scrape the sides of the bowl, then add the rest of the flour and milk. Beat just until fully combined.
In a small bowl, mix together the apple cider vinegar and baking soda until dissolved. Add to the batter and mix in on low speed, until fully incorporated.
Finally, add the Baileys Red Velvet and mix until smooth.
Fill each well of the cupcake pan with the batter to two-thirds full. Bake for 16 to 18 minutes (start checking at 15 minutes) or until a toothpick inserted into the center of a cupcake comes out clean. Transfer the pan to a wire rack and cool completely.
For the frosting
Place the butter, cream cheese, and vanilla extract in the bowl of a stand mixer (or use a hand-held electric mixer). Beat on medium speed to combine. Slowly add the confectioners’ sugar until incorporated.
Then beat on high speed for 3-5 minutes. Add the Baileys Red Velvet and mix on low speed until mixed in, then beat on high speed for an additional minute.
Transfer the frosting to a plastic piping bag fitted with a plain round metal tip and frost each cupcake.