Step 1

Preheat oven to 350F. Lightly grease a 9x5 loaf pan and line with parchment paper.

Step 2

In a large saucepan, melt the butter and 3 tablespoons peanut butter.

Step 3

Remove from the heat and whisk in cocoa powder, sugar, and salt (batter will be thick and somewhat grainy).

Step 4

Whisk in the egg and vanilla.

Step 5

Switch to a rubber spatula and stir in the flour and peanut butter chips. Spread batter into the pan.

Step 6

In a microwavable-safe bowl or cup, warm the marshmallow fluff for 15 seconds or until easy to spread. Spoon onto of the brownie batter and swirl around with a knife. Repeat with the remaining 2 tablespoons peanut butter.

Step 7

Spread into the pan. Bake 18-20 minutes or until the edges pull away from the sides and the center doesn't look wet.

Step 8

Cool to room temperature then remove from the pan. If brownies stick, refrigerate until cold, bring back to room temperature, then try again. Store in an airtight container for up to 1 week.