Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a large mixing bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, whisk together the vegetable oil, milk, eggs, and vanilla extract until combined.
Gradually add the wet ingredients to the dry ingredients, mixing until smooth.
Carefully stir in the boiling water until the batter is thin and well combined.
Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool completely in the pan on a wire rack.
While the cake cools, prepare the jelly cat layer: sprinkle the gelatin powder over the cold water in a small bowl and let it bloom for 5 minutes.
In a small saucepan, combine the mixed berry juice, sugar, and lemon juice. Heat over medium heat until the sugar dissolves and the mixture is hot but not boiling.
Remove from heat and stir in the bloomed gelatin until fully dissolved.
Pour the jelly mixture into a cat-shaped silicone mold or a shallow dish lined with a cat-shaped stencil. Refrigerate until set, about 2 hours.
Once the cake and jelly are fully set, carefully unmold the jelly cat and place it on top of the cooled chocolate cake.
Use black edible gel icing to draw the cat’s facial features and details on the jelly cat.
Slice and serve the chocolate cake jelly cat chilled or at room temperature.