Preheat the oven to 160°C (320°F) and line a 12-cup muffin tin with paper liners or set out mini cauldrons if you have them.
Sift the flour, sugar, cocoa powder, salt, baking powder, and baking soda into a large mixing bowl, then whisk until everything is well combined.
Add the egg, vegetable oil, and milk to the dry ingredients, then pour in the boiling water and stir gently until the batter is smooth without overmixing.
Divide the batter evenly among the liners, filling each about two-thirds full, then bake for 20 to 25 minutes until a toothpick inserted in the center comes out with just a few moist crumbs.
Remove the cupcakes from the oven, let them cool in the tin for a few minutes, then transfer to a wire rack and cool completely.
While the cupcakes cool, whip the cream cheese, cream, and sugar in a stand mixer fitted with a whisk attachment for about 5 minutes until the frosting is light and fluffy, then tint it with green food coloring and mix again until even.
Transfer the frosting to a piping bag fitted with a small round tip, then pipe blobs of frosting over the top of each cooled cupcake to look like a bubbling potion.
Place each cupcake into a mini cauldron or leave them in their liners, then decorate with candy eyes, lollipops, and gummy sweets to create a spooky, bubbling cauldron effect.
Keep the decorated cupcakes in the fridge until you are ready to serve.