After you open the can of cat food, carefully turn it upside down over a plate so the whole can is still in it's can shape.
With a large knife, carefully slice an inch off the top. Set that aside fully intact. (We'll use this later)
Cut another section of about 2-3 inches high and place that onto a festive birthday dessert plate.
Use a skinny glass like a champagne flute to make the cake topper. Turn the glass upside down and push into the middle of the top section of the cat food you sliced earlier. Remove the food around the outside of the glass and gently pull the glass off of the food. It should form a small circle of food you can place on the cake as your top layer.
Decorate with dry cat food and treats as you like.