Spread the vanilla frosting evenly over the tops of the four cooled chocolate cupcakes using a knife or offset spatula.

Place the chocolate chips and vegetable oil in a small microwave-safe bowl. Heat in 15-second bursts, stirring between each, until the chocolate melts into a smooth liquid.

Spoon the melted chocolate into a piping bag or a zip-top bag with one corner snipped off. Pipe three or four thin concentric circles on each frosted cupcake.

Use a toothpick to drag lines from the center outward through the circles, creating a spiderweb pattern on every cupcake. Wipe the toothpick clean between drags if needed.