Prepare the Baking Pan: Lightly grease a 9 x 13 inch baking pan with butter or nonstick spray, then lay parchment paper into the pan with overhang.

Preheat the Oven: Preheat your oven to 350°F (175°C) and adjust the rack to the center position.

Mix the Wet Ingredients: In a large bowl, combine melted butter and peanut butter. Gradually add sugars, then incorporate eggs and vanilla.

Combine the Dry Ingredients: In a separate bowl, whisk together flour, baking powder, baking soda, and fine sea salt.

Fold the Mixtures Together: Gently add dry ingredients into wet, folding until just combined.

Add the Chocolate Chips: Sprinkle in chocolate chips and mix gently until evenly distributed.

Spread the Dough into the Pan: Transfer dough into prepared pan and press down evenly.

Add Festive Toppings: Press M&M’s into the surface; sprinkle on Valentine's sprinkles if desired.

Bake the Cookie Bars: Bake in preheated oven for 18 to 24 minutes, checking for doneness.

Finishing Touches: Press reserved M&M’s on top and sprinkle with flaky sea salt.

Cool and Slice: Cool in the pan for 1 to 1½ hours; slice after lifting out with parchment.

Optional Drizzle: Combine white chocolate chips and oil, heat, then drizzle over cooled bars.

Storage Tips: Store in an airtight container at room temperature for 3-4 days or refrigerate up to a week.