Step 1: Preheat your oven to 350°F (175°C) and generously grease a 24-count mini muffin tin or line with paper liners.
Step 2: In a large bowl, whisk together the melted butter and granulated sugar until well combined and slightly cooled.
Step 3: Add the eggs and vanilla extract to the butter mixture, whisking vigorously for 1-2 minutes until the batter is smooth and shiny.
Step 4: Sift in the cocoa powder, flour, salt, and baking powder. Gently fold the dry ingredients into the wet ingredients using a spatula until just combined.
Step 5: Divide the batter evenly among the mini muffin cups, filling each about 3/4 full. Bake for 12-15 minutes or until a toothpick comes out clean.
Step 6: Allow the brownie bites to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before decorating.
Step 7: Place the mini marshmallows in a microwave-safe bowl and heat in 15-second intervals until melted and puffy. Stir until smooth.
Step 8: Let the marshmallow cool for about 30 seconds. Using your fingers, pull a small amount of marshmallow apart to create ‘web’ strands and drape them over the brownie bites.
Step 9: Add a plastic spider on top if desired and serve immediately or store in an airtight container.