Preheat the oven to 350 degrees Fahrenheit.
Spray each cavity of a 12-serving donut pan with non-stick cooking spray. Set aside. If using silicone donut pans, place them on a metal baking sheet for stability.
Add flour, baking powder, baking soda, and ½ cup cocoa powder to a mixing bowl and whisk to combine. Set aside.
Add butter, granulated sugar, vanilla extract, and eggs to a large mixing bowl and beat with an electric mixer until well combined.
Add the dry ingredients and 1 cup of milk to the mixing bowl and beat until smooth and creamy.
Spoon the batter into a large Ziplock bag, snip off one of the corners with scissors, and squeeze the batter evenly into each cavity of the prepared donut pan.
Bake for 17-18 minutes or until a toothpick inserted into one of the donuts comes out clean.
Remove the donuts from the baking pan and transfer them to a wire cooling rack to cool completely.
Add 2 cups powdered sugar and 4 tablespoons milk to a medium mixing bowl and mix until smooth. Add a few drops of green food coloring and mix until the food coloring is fully combined. Repeat if necessary until the desired shade of green is achieved.
Carefully dip the top of a cooled donut into the green glaze, turn it over, and place it back on the cooling rack. Repeat until all donuts are dipped in the glaze.
Top each donut with a small amount of sprinkles, and allow the glaze to dry while you make the spiders.
Add the remaining ½ cup of powdered sugar, 1 tablespoon cocoa powder, and 1 tablespoon milk to a small mixing bowl and mix until smooth.
Spoon the chocolate glaze into a small piping bag and snip off a very small corner with scissors
Pipe the spider legs onto each donut, and top with a chocolate truffle to form the spider.