Preheat oven to 350°F. Line 9x13 inch pan with foil and spray with cooking spray.
In medium sauce pan, melt butter. Once completely melted, remove from heat and stir in cocoa powder; set aside.
In large bowl, cream together eggs, sugar and vegetable oil until light and foamy. Add chocolate mixture and vanilla and stir until smooth.
In medium bowl, whisk together 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 until toothpick inserted into center comes out clean. Allow to cool completely before cutting into squares.
For the decorative icing, fill a small re-sealable bag with melted white chocolate. Cut off one corner and pipe chocolate onto brownies in shape of spider webs. Serve immediately, or store in a single layer in an airtight container.