I preheat my oven to 350°F (175°C) and line the pan with parchment paper. Leaving some paper hanging over the edges makes it so much easier to lift the bars out later.
I beat softened butter and sugar together with a mixer until it turns pale and fluffy. This step makes the bars lighter and prevents them from being too dense.
I crack in the eggs one at a time, mixing well after each addition. Then I add the vanilla, which instantly gives the batter that cozy, sweet aroma.
In another bowl, I whisk together flour, cocoa powder, baking powder, and salt. Then I slowly mix the dry ingredients into the wet mixture until I get a smooth batter.
I pour the batter into the pan and smooth it with a spatula. The bars bake for about 25–30 minutes, or until a toothpick comes out mostly clean with just a few crumbs.
While the bars are still warm, I scatter marshmallows, chocolate chips, and peppermint candy on top. The warmth melts everything slightly, making them look irresistible.
I let the bars cool fully in the pan before lifting them out with the parchment. Once cooled, I slice them into squares, and they’re ready to enjoy!