Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan. In a large mixing bowl, cream together butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract. In another bowl, whisk together flour, cocoa powder, salt, and baking powder. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Pour the batter into the prepared pan and spread evenly.

Melt the white chocolate chips and vegetable oil together in a microwave-safe bowl, stirring every 30 seconds until smooth. Transfer the melted chocolate to a piping bag or a resealable plastic bag with a small corner snipped off. Pipe concentric circles of white chocolate onto the surface of the brownie batter. Using a toothpick or skewer, drag through the circles from the center outwards to create a web pattern.

Place the pan in the preheated oven and bake for 25-30 minutes, or until a toothpick inserted into the center comes out mostly clean. Allow the brownies to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.

Once completely cooled, cut the brownies into slices. Serve them as a spooky centerpiece for your Halloween party or enjoy them as a delightful dessert after dinner.

Store any leftover brownies in an airtight container at room temperature for up to 3 days. For longer storage, you can refrigerate them for up to a week, ensuring they are wrapped well to maintain freshness.