Preheat your oven to 170°C (340°F). Grease a 9-inch square cake tin with butter or cooking spray, then dust with flour.

In a large mixing bowl, cream together 360 g of softened butter and 360 g of granulated sugar using an electric mixer on medium speed until light and fluffy, about 3 to 5 minutes.

Add 6 eggs to the creamed mixture, two at a time, beating well after each addition until combined.

In a separate bowl, whisk together flour, baking powder, black cocoa powder, and salt. Gradually sift this into the wet mixture in three parts, alternating with 90 ml of milk, folding gently until combined.

Pour the batter into the prepared tin, smooth the top, and bake for 45 to 50 minutes, or until a skewer inserted comes out clean.

Remove the cake from the oven, let it cool in the tin for 2 to 3 minutes, then transfer to a wire rack to cool completely.

In a clean bowl, cream 400 g of softened butter for the frosting, gradually add sifted powdered sugar, mixing on low speed, then whip until smooth and creamy.

Mix in 1¼ teaspoons of vanilla extract. If too stiff, add 2 tablespoons of milk or cream. Blend in electric green food coloring until uniform.

Once cooled, level the cake's top with a serrated knife and cut it into two layers. Use a round cutter to cut out 9 circles from each layer.

Sprinkle some sprinkles at the bottom of each jar, layer a cake circle, add a swirl of buttercream, more sprinkles, then top with another cake layer and buttercream swirl.

Create small holes in the top of the jars for whimsical fondant witch legs, securing them for the perfect Halloween effect.