Preheat your oven to 170°C (340°F) and prepare a 9-inch square cake tin by greasing it with butter and dusting it with flour.

In a large mixing bowl, cream together the butter and granulated sugar using an electric mixer until light and fluffy, about 3-5 minutes. Add eggs two at a time, mixing until well combined.

In a separate bowl, combine self-raising flour, unsweetened black cocoa powder, baking powder, and salt. Sift together and gradually add to the creamed mixture, alternating with milk and folding in vanilla extract.

Pour the cake batter into the prepared tin, spreading it evenly. Bake for 45-50 minutes or until a toothpick comes out clean. Allow cooling in the tin for 10 minutes before transferring to a wire rack.

Once cooled, slice the chocolate cake horizontally into two layers and cut 9 circles from each layer using a round cutter.

In a mixing bowl, beat softened butter until creamy, gradually adding in powdered sugar. Mix until fluffy, then add green food coloring to your desired shade.

To assemble, sprinkle a layer of colorful sprinkles at the bottom of each jar, followed by a layer of cake, then buttercream. Repeat until jars are filled.

Pipe additional buttercream on top of each jar and add sprinkles. Insert pre-made fondant witch legs into the buttercream to finish.