Preheat your oven to 325°F (160°C) while you prepare your baking pan. Grease an 8x8-inch square pan thoroughly with butter or oil, then line it with parchment paper, leaving an overhang for easy removal later.

In a large mixing bowl, whisk together the large eggs, vegetable oil, water, vanilla extract, granulated sugar, and kosher salt until fully combined and smooth. Sift in the all-purpose flour, Dutch-process cocoa powder, and powdered sugar, mixing gently until no dry streaks remain.

Pour the brownie batter into the prepared pan and use a spatula to spread it evenly. Place it in the preheated oven and bake for about 40 minutes, or until the brownies are set and a toothpick inserted in the center comes out with a few moist crumbs.

In a mixing bowl, combine the sweetened condensed milk, peppermint extract, and half the powdered sugar. Blend until it forms a crumbly paste, then roll it out between two sheets of parchment paper into an even square that matches the brownie base.

Once the brownies are completely cool, gently peel off the top layer of parchment paper from the peppermint filling. Place the rolled peppermint layer on top of the brownies, pressing it down lightly to adhere.

In a small saucepan, heat the heavy whipping cream over medium heat until it’s steaming but not boiling. Remove it from the heat and pour it over the chopped semi-sweet chocolate in a mixing bowl. Let it sit for about 5 minutes to soften, then stir until smooth and glossy.

Carefully spread the ganache over the peppermint layer using a spatula, ensuring an even coat that covers all edges. Refrigerate the assembled brownies for at least 30 minutes to set the ganache.

When ready to serve, use a warm knife to slice the Chocolate Peppermint Brownies into squares.