Preheat oven to 350° F. Line 12-cup muffin tin with cupcake liners. In a medium bowl, combine flour, sugar, baking soda, and salt; set aside.
In a medium saucepan combine water, butter, and cocoa powder. Bring mixture just to boiling, stirring constantly. Remove from heat. Add the cocoa mixture to the flour mixture. Beat with an electric mixer on medium speed until combined. Add the egg, buttermilk, and vanilla. Beat for 1 minute (batter will be thin). Pour batter into prepared muffin pan.
Bake about 20 minutes, or until a wooden toothpick inserted in the center comes out clean.
As the cupcakes cool, make the frosting. Place chocolate in a shallow bowl. Add the heavy cream to a small saucepan over medium-high heat until just simmering. Pour over the chocolate and let sit one minute before stirring until smooth. Let cool to room temperature; you can speed up the process by moving to the fridge and stirring every few minutes. Add the bowl of a stand mixer and beat with a whisk attachment until thick and fluffy.
Once cupcakes are at room temperature, pipe with chocolate ganache frosting. Top with assorted candies.