a pastry bag fitted with a medium round tip; a small piping bag
For the cupcakes: Position an oven rack in the middle of the oven and preheat to 350 degrees F. Place cupcake liners in two cupcake pans.
Whisk together the flour, granulated sugar, baking soda and salt in a medium bowl; set aside. Stir together the hot water and cocoa powder in a small bowl until smooth; let cool slightly. Stir together the milk and vinegar in another small bowl and let stand until thickened, 3 to 5 minutes.
Whisk together the oil, eggs and vanilla in a large bowl until smooth. Whisk in the red food coloring and the cocoa mixture. Alternate whisking in the flour mixture in three additions and the milk in two, starting and ending with the flour, until just combined. Divide the batter evenly among the cupcake liners.
Bake until a toothpick inserted in the center comes out clean, 15 to 18 minutes. Let cool for 10 minutes in the pans, then remove to cooling racks and let cool completely.
For the frosting: Meanwhile, beat the cream cheese, mascarpone, butter, vanilla and salt with an electric mixer on medium speed in a large bowl until smooth and fluffy. Gradually beat in the confectioners' sugar until smooth. Increase the speed to high and beat until very fluffy and there is no gritty texture left from the sugar. Beat in the milk to adjust the consistency if the frosting seems too thick.
Transfer the frosting to a pastry bag with a medium round tip. Pipe a circle onto each of the cupcakes.
Meanwhile, melt the chocolate in a microwave-safe bowl in the microwave or in a double boiler. Transfer the chocolate to a small piping bag and snip a tiny hole. Pipe a spiral on each of the cupcake tops that spans the full size of the top. Run a toothpick from the center point of the spiral and drag to the edge of the cupcake. Do this 3 to 5 times, creating a web. Repeat with the remaining cupcakes. Top with a gummy spider, if using. Scary!