This is the best chocolate sheet cake I've ever had! It is made with double to cocoa of the traditional chocolate cheese cake, so it's perfectly rich, and it's deliciously moist and fluffy, and who can resist that chocolate icing? It tastes like fudge and it's just too good!
Preheat oven to 350 degrees F. Butter a rimmed 18 by 13-inch baking sheet (average size cookie sheet) and dust lightly with cocoa powder and tilt to evenly coat, set aside.
In a medium mixing bowl whisk together flour, sugar, baking soda and salt for 20 seconds, set aside.
Add butter to a large microwave safe mixing bowl. Heat in microwave until melted. Whisk in cocoa powder and water.
Pour flour mixture into butter mixture and using an electric hand mixer, blend until well combined. Add in buttermilk, eggs and vanilla and mix on low speed for about 45 seconds.
Pour mixture into prepared baking dish. Bake in preheated oven until toothpick inserted into center of the cake comes out clean, about 20 - 25 minutes.
Remove from oven and set on a wire rack to cool, immediately begin to make icing (you want to ice the cake while it's still really warm so it stays really moist and sort of absorbs some of the icing).
Make the Chocolate Icing
Add butter, cocoa powder, salt and milk to a medium saucepan. Cook over medium heat, stirring constantly, until butter has melted.
Remove from heat, whisk in vanilla and powdered sugar (if mixture is thick return to medium-low heat and warm, stirring constantly, until it has thinned. It should be a nice pourable consistency).
Immediately pour icing over warm cake and spread into an even layer. If using sprinkles add immediately before icing begins to set.
Allow icing to set at room temperature. Cut into squares, if using chocolate whipped cream add just before serving and a strawberry to each slice.
If you'd like to make the chocolate whipped cream: whip 3/4 cup + 2 Tbsp heavy cream in mixing bowl using an electric hand mixer until soft peaks form. Add in 2 1/2 Tbsp cocoa powder, 3 1/2 Tbsp granulated sugar and 1/2 tsp vanilla and whip to stiff peaks. Chill until ready to use.