Preheat oven to 350°F. Line a 12-count muffin tin with cupcake liners, set aside.
In a medium saucepan over low heat, add the butter, oil, and water. Stir to combine until butter has melted. Remove from heat, set aside.
In a large bowl, sift the flour, sugar, cocoa powder, baking soda, salt, and coffee powder.
Add melted butter mixture to dry ingredients. Using a handheld mixer beat on low speed until combined.
Add egg and beat until incorporated. Add the buttermilk and vanilla then beat until smooth. Scrape down the sides of the bowl as needed. Batter should be slightly runny. Add batter to a liquid measuring cup.
Pour batter into prepared muffin tin, about ¾ of the way full. Bake for 20 minutes, or until a cake tester inserted comes out clean and tops are springy. Let cool then transfer to a wire rack to cool completely.
Buttercream Frosting
In a large bowl, beat butter and marshmallow cream together until smooth. Add vanilla extract, heavy cream, and salt and beat to combine.
With mixer on low, slowly add the powdered sugar. Continue to beat the icing on medium-high speed until light and fluffy, about 1 minute. Add more powdered sugar if frosting is too thin, or more cream if frosting is too thick. Add a pinch of salt if frosting is too sweet.
Add frosting to piping bag, fitted with a large tip, or with a ½” hole cut in the bottom. Pipe frosting onto cooled cupcakes in a swirling motion, then decorate with candy eyeballs. Serve immediately or store in an airtight container in refrigerator until ready to serve.