Line an 8 inch square pan with parchment, leaving an overhang for easy lifting. Lightly mist with cooking spray. Preheat the oven to 350°F.

Microwave butter in a heatproof bowl in short bursts until melted. Let it cool for 5 minutes so it does not scramble the eggs.

Whisk the melted butter with the brown sugar and optional granulated sugar until glossy. Add eggs and vanilla and whisk until thick and smooth.

In a separate bowl, whisk flour, baking powder, salt, cinnamon, and cayenne pepper (optional or pinch for mild).

Fold dry ingredients into the wet mixture using a spatula just until combined. Fold in chocolate chips, white chocolate chips, and chopped candy.

Scrape batter into prepared pan and smooth the top with spatula.

Optional devil swirl: Stir cocoa powder with sugar and water to form paste. Drizzle thin lines over surface and drag toothpick to create a web effect.

Bake for 22 to 26 minutes until edges are set and center is puffed and golden with moist crumbs on toothpick.

Cool on rack for at least 45 minutes. Lift out bars by parchment and cool completely to set bars for clean slices.

For red drizzle, tint melted white chocolate with red gel color and flick or pipe lines across surface. Press candy eyes on while drizzle is tacky.

Slice into 16 small squares or 9 large squares with a sharp knife wiping blade between cuts. Serve warm or at room temperature.