Combine all ingredients in a cocktail shaker and fill it three quarters with ice.
Shake until chilled, then strain into a margarita or cocktail glass.
For the Spider Web Garnish:
Place 1-2 marshmallows on a plate or in a bowl and microwave for 20-30 seconds.
Use a nitrile glove to pull the melted marshmallow apart, creating web-like strands. Stick one end of a wispy strand on the rim of your glass and pull across to the other side of the glass.
Continue pulling strands across the top of the glass to create a web-like design.
If the marshmallow begins feeling stiff or starts pulling too much on the other strands you've created, stop. You can always heat up another marshmallow if you want to add more webbing. Rinse and dry the plate or bowl after melting each batch of marshmallows to avoid hard, burnt on bits that can ruin the look of your web.