01 - Preheat oven to 350°F (175°C). Thoroughly grease and flour one 9-inch round cake pan and one 9x5-inch loaf pan.
02 - In a substantial mixing bowl, thoroughly whisk together the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking powder, baking soda, and salt.
03 - Incorporate the eggs, whole milk, vegetable oil, and vanilla extract into the dry ingredients. Beat on medium speed for two minutes. Carefully stir in the boiling water; the batter will be thin.
04 - Divide the batter evenly between the prepared round and loaf pans. Bake for 28 to 32 minutes, or until a wooden skewer inserted into the center emerges clean.
05 - Allow the cakes to cool in their pans for 10 minutes before inverting them onto wire racks to cool completely.
06 - In a separate bowl, beat the softened unsalted butter until it achieves a creamy consistency. Gradually incorporate the powdered sugar and cocoa powder, beating thoroughly after each addition. Mix in the milk, vanilla extract, and salt until the frosting is smooth and has a fluffy texture.
07 - Position the round cake on a serving platter to serve as the rat's body. Slice one end of the loaf cake into a point to create the nose, and attach it to the body. Utilize any remaining loaf cake pieces to fashion the rat's haunches or the base of its tail as desired. Generously frost the entire cake assembly with the chocolate buttercream, smoothing it to define the rat's form.
08 - Halve the marshmallows horizontally and tint the cut sides pink; affix them to the cake to represent ears. Add the chocolate candies or edible eyes for the eyes. Position a small pink candy as the nose. Arrange black licorice laces or thin pretzel sticks to mimic whiskers and a tail. Enhance with additional candies for a festive finish.
09 - Refrigerate the decorated cake until it is ready to be served.