Make the Cupcakes: Preheat oven to 350°F and line muffin pan with cupcake liners.
Whisk soy milk and vinegar in a measuring cup and set aside for a few minutes to curdle.
Beat together the soy milk mixture, oil, sugar, vanilla in a large bowl. Sift in the flour, cornstarch, baking powder, baking soda and salt and mix until no big lumps remain.
Fill cupcake liners ⅔rds of the way full. Sprinkle the chopped candy on the batter and bake for 20-22 minutes or until a toothpick inserted in the center comes out clean.
Transfer to a cooling rack and let them cool completely before frosting.
Make the Frosting: In a small saucepan, combine sugar, cocoa, milk, butter and corn syrup. Bring this mixture to a boil on medium flame, stirring constantly. Heat for 3 minutes. Remove from heat and stir in vanilla and add enough confectioner's sugar to make a spreadable frosting.
Frost the cupcakes and garnish with some m&m's.