Preheat oven to 350°F. Line cupcake trays with paper liners.
Cream butter and caster sugar together until smooth.
Whisk egg. Add half of the egg into creamed mixture. Mix until uniform. Add remaining egg and repeat.
Sift flour into mixture and fold together gently.
Portion batter into cupcake molds, filling about halfway.
Bake cupcakes for around 20 minutes (until golden brown).
Move to wire rack for cooling.
Icing
Sift powdered sugar into a bowl and mix with warm water. Add more powdered sugar or water as needed until icing is desired consistency. Add green food coloring until icing is desired color.
Frost cupcakes.
Garnish
Garnish with chocolate mice and chocolate chips as shown in photo.
Notes
Caster sugar is a finer grain of sugar than granulated. Granulated sugar can be ground in a coffee grinder or food processor if caster sugar is unavailable.
Self-rising flour: 1 cup all-purpose flour, 1 1/4 teaspoon baking powder, a pinch of salt