In a big bowl, first whisk flour, sugar, cocoa, baking soda, salt. Add milk, oil, vinegar, vanilla, and peppermint. Whisk until smooth. Fold in chocolate chips if using.
Fill liners ⅔ full. Then bake 18–22 minutes or until the tops spring back. Be sure to cool completely.
For the frosting
Beat butter until fluffy then add powdered sugar gradually.
Add peppermint + milk until smooth.
Pipe tall swirls and sprinkle crushed candy canes on top.
Notes
Be careful not to overmix the batter. Once it all comes together, stop whisking. A few small lumps are cool.
Peppermint extract is strong. Start with a lower amount if you're unsure. this can go from festive to toothpaste easily.
Cool completely before frosting. This is good advice for any cake.