Preheat your oven to 170°C (340°F) and grease a 9-inch square cake tin with butter or cooking spray.

In a large mixing bowl, cream butter and sugar until light and fluffy (3-5 minutes).

Add eggs two at a time, mixing well for about 2 minutes.

In another bowl, sift flour, cocoa powder, baking powder, and salt. Gradually combine with the wet mixture, alternately adding milk.

Pour the batter into the prepared tin and bake for 45-50 minutes. Let cool in the tin for 10 minutes.

Transfer cake to wire rack and cool completely for at least 1 hour.

Cream butter until smooth, then gradually add powdered sugar. Mix in milk and vanilla, then stir in food coloring.

Use a round cutter to slice the cooled cake into circles.

Layer the jars starting with sprinkles, then cake, and buttercream. Repeat layers until full.

Pipe a swirl of buttercream on top and insert fondant witch legs for decoration.