Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9-inch square pan.

Stir sugar, 3/4 cup plus 2 tablespoons flour, cocoa powder, baking soda, baking powder, and salt together in a large bowl. Add milk, vegetable oil, egg, and vanilla extract; beat with an electric mixer on medium speed until batter is smooth. Beat in 1/2 cup boiling water. Pour into the prepared baking pan.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Let cool in the pan for 10 minutes. Invert onto a wire rack to cool completely, about 30 minutes.

Cut marzipan into 3 equal pieces. Add varying amounts of green food coloring to each piece to get different shades of green. Knead each piece separately to incorporate food coloring evenly.

Dust a flat work surface with 1 tablespoon confectioners' sugar. Roll out each piece of marzipan into a thin rectangle. Cut into equal-sized squares.

Combine 1/3 cup confectioners' sugar and 1 tablespoon water in a shallow dish; stir, adding more water if needed, until mixture is the consistency of cream.

Dip the back of a marzipan square in the confectioners' sugar mixture and place on the outer edge of the cake. Arrange remaining squares, alternating the 3 shades of green, randomly over the cake from the outer edges to the center. Leave 3 squares of chocolate cake exposed in the center for the Creeper's eyes and mouth. Cut into marzipan on either side of the mouth to create its characteristic shape.