Add your semisweet chocolate chips, sweetened condensed milk, and peppermint extract to a large microwavable bowl. Place in the microwave for 30 seconds at a time mixing between until fully melted. This could have 2-3 minutes to cook.
Once everything is melted mix until well combined. Pour these ingredients into an 8x8 casserole dish (I used parchment paper for easy removal). Top with peppermint crunch baking chips.
Place in the fridge until set. This will take at least 2 hours. Then slice into fudge pieces and enjoy!