In a large bowl, cream together the softened unsalted butter and granulated sugar until light and fluffy. Beat in the vegetable oil until combined.

Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract and peppermint extract.

In a separate bowl, whisk together the Dutch process cocoa powder, baking powder, and kosher salt.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.

Fold in the dark chocolate chips or chopped bar. Cover the dough and chill in the refrigerator for at least 30 minutes, or up to 2 hours.

Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper. Scoop rounded tablespoons of dough onto the prepared baking sheet, leaving some space between them.

Bake for 9-11 minutes, or until the edges are set but the centers are still slightly soft. Let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.