Preheat the oven to 350ºF. Line 20 muffin cups with paper liners. Set aside.
Using a handheld or stand mixer fitted with a paddle attachment, beat the oil, butter, and sugars at medium-high speed until fluffy, about 3 minutes.
Add the eggs one at a time mixing well after each addition. Beat in the vanilla.
In a separate bowl, combine the flour, cocoa powder, baking soda, baking powder, and salt. Stir with a whisk. All half the flour mixture to the butter mixture. Mix at low speed until almost combined. Add the milk followed by the remaining flour mixture and mix until well blended.
Fill the prepared muffin cups half full, about 2 tablespoons of batter each. Bake for 15-20 minutes or until a wooden toothpick inserted into the center comes out with only a few dry crumbs. Remove cupcakes immediately from pans, and cool completely on a wire rack.
Make the frosting
Using a handheld mixer and a large bowl or stand mixer fitted with a paddle attachment, combine the butter, sugar, cream, and peppermint extract. Beat on medium speed until smooth.
Add the food coloring. Beat until blended. Top the cupcake with the frosting. Decorate with chocolate mint candies if desired.
Notes
Mint extract: Some may find that green mint exact is overpowering even though it's true to the Andes mint flavor. You may prefer to use peppermint instead as it's not so strong.
Filling muffin cups: This recipe will rise significantly. It's important not to fill the muffin cups more than half full or the batter will spill over the sides as it bakes and rises. Muffins cups hold 4 tablespoons of batter. I suggest only using 2 to fill the cups. If you have a 2-tablespoon cookie scoop, it will be helpful in portioning the batter.
Make ahead tip
The cupcakes can be made 1 day in advance before frosting. Cover and refrigerate each overnight. Frost the cupcakes the next day.
Or the cupcakes can be frozen for up to 3 months. Thaw overnight in the refrigerator and frost as desired. Keep frosted cupcakes covered and store them in the refrigerator for up to three days. Bring it to room temperature just before serving.
Frosting will keep for up to 2 weeks stored in the refrigerator in an airtight container. You must let the buttercream come to room temperature by setting it out on the counter for a couple of hours, then beat it with an electric mixer if needed.
Or the frosting can be frozen for up to 3 months. When ready to use, place in the refrigerator overnight to thaw. Let stand at room temperature to soften before use.