Preheat your oven to 350°F (175°C). This ensures your bars bake evenly.
In a mixing bowl, combine the softened butter, brown sugar, and granulated sugar. Use an electric mixer to cream them together until the mixture is light and fluffy, about 2-3 minutes.
Mix in the vanilla and milk until fully incorporated.
Gradually add the cocoa powder and flour to the mixture. Mix until just combined, being careful not to overmix, as this can lead to tough bars.
Gently fold in the white chocolate chips and creamy peanut butter until evenly distributed throughout the dough.
Line your baking pan with parchment paper for easy removal. Spread the cookie dough evenly into the pan, pressing it down gently.
Place the pan in the preheated oven and bake for 25-30 minutes, or until the edges are golden brown. The center might look slightly underbaked, but it will firm up as it cools.
Allow the bars to cool in the pan for about 10 minutes, then transfer them to a wire rack to cool completely. Once cool, slice into squares and enjoy your delicious creation!
