Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

Cream together the softened butter and sugar until light and fluffy using an electric mixer.

Beat in the eggs one at a time, then stir in the vanilla.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.

Gently fold in the chocolate chips (if using). Pour batter into the prepared pan and spread evenly.

Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs.

Let cool completely. Melt white chocolate. Transfer to a piping bag. Pipe concentric circles, then drag lines from the center outwards to create a spiderweb.

Let the spiderweb set before cutting and serving.