Place 1 marshmallow horizontally on a work surface. Using a rolling pin or soup can, flatten the marshmallow by rolling back and forth over it. Using Halloween-themed mini cookie cutters, cut into desired shapes.
In a saucepan over medium heat, combine the cocoa, sugar, and water. Bring slowly to a boil, stirring, until the sugar and cocoa dissolve, about 2 minutes. Add the milk and the half-and-half and continue to cook, stirring, until bubbles appear along the edges of the pan, about 5 minutes; do not allow to boil. Pour into mugs and garnish with the marshmallow spooks. Serve immediately while the spooks hold their shapes.