01 - Preheat oven to 350°F. Line a muffin pan with 12 cupcake liners to ensure even baking.
02 - Whisk together the flour, cocoa powder, baking powder, and salt in a mixing bowl until fully blended.
03 - In a separate large bowl, beat unsalted butter and granulated sugar until light and airy. Add eggs one at a time, followed by vanilla extract, mixing after each addition.
04 - Gradually mix dry ingredients into the butter mixture, alternately with the milk. Begin and finish with dry ingredients, stirring until just combined.
05 - Portion the batter evenly into cupcake liners, filling each about two-thirds full. Bake 16 to 18 minutes until a toothpick inserted into the center comes out clean.
06 - Transfer cupcakes to a wire rack and cool completely before decorating.
07 - Beat butter until creamy. Sift in powdered sugar and cocoa powder, then add milk, vanilla extract, and a pinch of salt. Continue beating until frosting is smooth and fluffy.
08 - Generously spread chocolate frosting onto each cooled cupcake using a spatula or piping bag.
09 - In a microwave-safe bowl, heat mini marshmallows for 15 to 20 seconds until melted. Stir until smooth, let cool until stretchy yet safe to handle. Pinch off small portions and stretch between your fingers, then drape over frosted cupcakes to mimic spider webs.
10 - Optionally, embellish with candy spiders, edible glitter, or themed decorations as desired.