Preheat the oven to 350°F (175°C). Grease and flour a cat-shaped cake pan or a round cake pan if a shaped pan is unavailable.
In a large mixing bowl, sift together the all-purpose flour, granulated sugar, cocoa powder, baking soda, and salt.
In a separate bowl, whisk together the water, vegetable oil, white vinegar, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Add the eggs one at a time, mixing gently after each addition until incorporated.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for 10 minutes. Then transfer it to a wire rack to cool completely.
While the cake cools, prepare the chocolate frosting. In a medium bowl, beat the softened butter until creamy.
Gradually add the powdered sugar and cocoa powder, alternating with the whole milk, beating until smooth and spreadable.
Stir in the vanilla extract.
Once the cake is completely cooled, spread the chocolate frosting evenly over the cake, shaping it gently to enhance the cat features if using a shaped pan.
For the jelly glaze, combine the fruit jelly and water in a small saucepan over low heat. Stir until melted and smooth.
Using a pastry brush, gently brush the jelly glaze over the frosted cake to give it a shiny, glossy finish.
Allow the jelly to set for 10 minutes before serving.