Step 1

PREHEAT oven to 350F. Spray a 9 x 13-in. baking pan and line with parchment up the length of 2 sides.

Step 2

WHISK flour, sugar, cocoa, baking soda, baking powder and salt in a medium bowl.

Step 3

WHISK eggs, buttermilk, oil and vanilla in a large bowl. Stir in flour mixture until just combined. Whisk in hot water. Pour into prepared pan.

Step 4

BAKE until a cake tester inserted in centre of cake comes out clean, about 30 min (start checking for doneness at 25 min). Let cake cool in pan on a rack for 30 min. Place rack on top of cake, then invert and remove baking pan and parchment. Cool completely, about 30 min. Invert onto serving platter.

Step 5

STIR icing sugar and cocoa in a medium bowl. Beat butter in a large bowl with a hand mixer on medium-high, until fluffy. Reduce speed to medium-low and add icing sugar mixture, alternating with milk and ending with icing sugar mixture, until combined. Spread on chocolate cake and sprinkle with candy.