Heat oven to 350°F. Line 12-cup muffin pan with baking cups.
Sift together sugar, flour, cocoa, baking powder, baking soda, and salt in large bowl. Add eggs, milk, oil, and vanilla; beat on medium speed of mixer for 2 minutes. Stir in boiling water (batter will be thin). Fill baking cups.
Bake for 18 minutes or until wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pan to rack. Cool completely before frosting.
In a small pan, heat cream over low-medium heat until heated through. Remove from heat. Add chocolate and stir until blended. Let cool before frosting cupcakes. Add about 1 teaspoon of frosting to each cupcake. Top frosted cupcakes with Halloween candy sprinkles.