01 - Preheat oven to 350°F. Grease a 9x5-inch loaf pan and line with parchment paper.
02 - In a large bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
03 - In a separate bowl, blend melted butter and sugar until smooth. Add eggs one at a time, mixing thoroughly after each addition. Stir in milk and vanilla extract.
04 - Pour wet mixture into dry ingredients and stir gently until just combined. Do not overmix.
05 - Carefully fold in chocolate chips, rainbow sprinkles, marshmallows, and nuts if desired.
06 - Spread half of the batter into the prepared pan. Dollop half of the peanut butter on top and gently swirl with a knife. Repeat with remaining batter and peanut butter, swirling again.
07 - Bake 50 to 55 minutes, or until a toothpick inserted in the center comes out with moist crumbs.
08 - Allow loaf to cool in the pan for 15 minutes. Transfer to a wire rack and cool completely before slicing.