Preheat the oven to 350°F and grease an 8-inch round cake pan. Line the bottom with parchment paper.
In a large bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, and salt.
In a separate bowl, mix the buttermilk, vegetable oil, egg, 1 teaspoon vanilla extract, and black food coloring until smooth.
Pour the wet mixture into the dry ingredients and stir until just combined. Do not overmix.
Pour the batter into the prepared pan and bake for 25 to 30 minutes, or until a toothpick comes out clean. Cool the cake completely in the pan.
While the cake cools, beat the cream cheese and butter together until creamy. Add the powdered sugar and remaining teaspoon of vanilla extract, then beat until fluffy.
Remove the cooled cake from the pan and spread the cream cheese frosting evenly over the top and sides.
Use the black decorating gel to pipe a spiderweb design on the frosted surface, starting with concentric circles and dragging lines outward with a toothpick.