Preheat your oven to 170 degrees Celsius. Grease and flour a 9-inch square cake tin.

In a large mixing bowl, combine 360 g of softened butter and 360 g of granulated sugar. Cream together for about 2 minutes until light and fluffy.

Crack in the eggs two at a time, mixing well after each addition. This should take about 4-5 minutes.

In a separate bowl, whisk together the flour, baking powder, cocoa powder, and salt.

Sift the dry ingredients into the creamed ingredients in three batches, alternating with milk.

Pour the batter into the prepared cake tin and bake for 45 to 50 minutes.

Let the cake rest in the tin for 2 to 3 minutes before transferring it to a wire rack to cool completely.

In a clean mixing bowl, cream 400 g of softened salted butter until luscious.

Gradually sift in 800 g of powdered sugar. Mix on low until moistened, then whip until creamy.

Mix in 1¼ teaspoons of vanilla extract and add electric green food coloring until desired hue is reached.

Level the top of your cooled cake with a serrated knife and slice it horizontally into two equal layers.

Cut out 9 circles from each of your two cake layers.

Add a spoonful of sprinkles to the bottom of each jar, insert the first cake layer, and pipe a generous layer of buttercream.

Repeat by adding sprinkles to the sides, then another cake layer topped with buttercream.

Pipe a swirl of buttercream on top and sprinkle with additional sprinkles.

Insert decorative fondant witch legs into the jars.