Spray a quarter sheet pan lightly with cooking spray. Line the sheet with parchment paper, then spray the top of the paper with a light even coating of cooking spray.
Finish prepping the sheet pan by giving it a light sprinkle of flour all over.
Add the sugar, milk, oil, vanilla, and eggs to a large mixing bowl. Use a hand or stand mixer to mix the wet ingredients until well combined, about one minute.
Add the flour, baking powder and salt to the bowl. Mix the dry and wet ingredients together on low speed until they form a smooth creamy batter.
Pour the chocolate chips into the bowl. Use a rubber spatula or wooden spoon to gently fold the chocolate chips into the batter by hand.
Use a large spoon to dollop all of the cake batter evenly over the prepared quarter sheet pan. With an offset spatula, or the back of a spoon, smooth the top of the cake batter inside the sheet pan.
Place the cake in the preheated oven and bake it for 24 to 27 minutes, or until the edges of the cake are golden and beginning to pull away from the sides of the pan. The top of the cake may look pale after baking, but should feel firm to the touch of a fingertip.
Remove the sheet cake from the oven and cool it on a wire rack before decorating it with frosting (optional). You can leave the cake in the sheet pan to serve if you like, or you can remove it and serve it on a platter or overturned baking sheet.
The recipe as written bakes up as a low-profile flat cake. If you like your sheet cakes to have a bit more lift, add an extra teaspoon of baking powder.Adding flavours to the cake itself will kick this basic old-fashioned recipe up a notch or two. Some ideas:
Add the zest of orange to the cake batter, and substitute ⅓ cup of milk for orange juice.
Use one or two teaspoons of spices like ginger, homemade chai spice, cinnamon, or pumpkin pie spice in the dry ingredient mix.
Add ¼ cup of rainbow sprinkles to the cake. Because who can resist sprinkles?!
Make a chocolate cake by adding ¼ cup of cocoa powder to the dry ingredients, and ¼ cup of melted chocolate to the wet ingredients.
SUBSTITUTIONSYou can substitute the oil for butter if you like. Use softened butter and cream it together with the sugar before adding the milk, vanilla, and eggs.Light spelt flour can be substituted for all-purpose at a 1:1 ratio for a whole grain option.Use any chocolate chips you like. You can also use chopped chocolate, however, it won't hold it's structure during baking the way chocolate chips do.If you don't own a quarter sheet pan, bake this recipe in two 8-inch (20-cm) cake tins for about 30 to 35 minutes, or until golden, firm to the touch on top, and the edges begin to pull away from the sides of the pan.Substitute dairy-free milk and use dark chocolate, or vegan chocolate chips for a dairy-free cake.