Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners.

Whisk the flour, cocoa powder, baking soda, baking powder, and salt together in a medium bowl. Set aside.

In a large bowl, whisk the oil, sugar, egg, vanilla, and buttermilk together until combined. Pour the dry ingredients into the wet ingredients, add the hot coffee/water, and whisk until the batter is completely combined.

Pour the batter into the liners, filling only ⅔ full to avoid spilling over the sides.

Bake for 20–22 minutes, or until a toothpick inserted in the center comes out clean.

Allow cupcakes to cool in the pan for 10 minutes, and then transfer to a wire rack to cool completely.

With a handheld or stand mixer, beat the butter on medium speed until creamy, about 2 minutes.

Add the confectioners’ sugar, heavy cream/milk, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes.

Taste and add a pinch of salt if frosting is too sweet. Adjust the consistency by adding more confectioners' sugar or cream/milk as needed.

Frost cooled cupcakes and top with sprinkles if desired.

Store leftover cupcakes covered at room temperature for 1 day or in the refrigerator for 3 days.