Preheat oven to 350°. Line a 9x13 inch pan with foil and spray with cooking spray.
In a medium sauce pan, melt butter. Once melted, remove from heat and stir in cocoa powder; set aside.
In large bowl, beat together eggs, sugar and vegetable oil until light and foamy. Add butter/cocoa mixture and vanilla. Stir until smooth.
In medium bowl, mix flour, baking powder and salt. Add dry ingredients to wet ingredients and stir until combined. Pour mixture into prepared pan.
Bake for 25-30 minutes. A toothpick inserted into center should come out clean. Allow to cool completely before cutting into squares.
For the decorative spiderwebs, fill a small ziplock bag with melted white chocolate. Cut off one corner and pipe chocolate frosting onto brownies in the shape of spider webs. Serve immediately, or store in an airtight container for up to 3-5 days.