In a microwave-safe bowl, combine the granulated sugar, softened butter, and water. Microwave in 45-second increments, stirring in between, until the butter is melted and the mixture is hot.

Add the 1 cup semi-sweet chocolate chips to the warm mixture and stir until the chocolate is completely melted and smooth. If needed, microwave for a few more seconds and stir again.

Stir in the egg and vanilla extract until the mixture is well combined.

Add the all-purpose flour and baking soda, mixing just until the batter is combined and smooth. The batter will be thick and glossy.

Allow the batter to cool slightly at room temperature for about 5–10 minutes so it is not too hot when adding the milk chocolate chips.

Fold in the 3/4 cup milk chocolate chips until evenly distributed.

Preheat the oven to 350°F (175°C). Grease a muffin tray or line with paper liners. Spoon the batter into the muffin cups, filling each about 3/4 full.

Bake for 14–16 minutes, or until the tops look set and a toothpick inserted near the edge of a brownie comes out mostly clean with a few moist crumbs.

Remove the muffin tray from the oven and let the brownie bites cool in the pan for about 5 minutes. If the centers do not naturally sink, gently press the center of each brownie with the back of a tablespoon to create a small well.

Place the creamy peanut butter in a microwave-safe bowl and microwave in short bursts until melted and smooth.

Spoon about 1 tablespoon of melted peanut butter into the center of each brownie bite, filling the well. Top with additional chocolate chips if desired.

Allow the brownie bites to cool until the peanut butter has thickened slightly. Serve warm or at room temperature.