Preheat your oven to 350°F (175°C). This ensures your brownie cooks evenly.
In your cast-iron skillet, melt the 1 cup of unsalted butter over low heat. Stir gently until fully melted, then remove from heat and let it cool slightly.
Add 1 cup of sugar and 1 cup of dark brown sugar to the melted butter. Mix until well combined.
Add the lightly beaten eggs and 1 tablespoon of vanilla extract to the butter and sugar mixture. Whisk until smooth and well incorporated.
In a separate bowl, sift together 1 cup of all-purpose flour, 3¼ cup of unsweetened cocoa powder, 1 tablespoon of instant espresso powder, and 1 teaspoon of salt. Gradually fold this dry mixture into the wet ingredients until just combined. Be careful not to overmix.
Fold in 1 cup of chocolate chips and ½ cup of creamy peanut butter until evenly distributed throughout the batter.
Spread the brownie batter evenly in the skillet. For an extra touch, sprinkle more chocolate chips on top. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownie to cool in the skillet for about 10 minutes. Serve warm, preferably with a scoop of vanilla ice cream on top. Enjoy this delightful Chocolate Peanut Butter Skillet Brownie!