1-Preheat the oven to 350°F (177-180°C).

2-Grease a 6-inch round cake pan and line the bottom with parchment paper; grease the parchment as well for easy release.

3-In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder (if using), espresso powder, and salt.

4-In another bowl, whisk the vegetable oil, granulated sugar, egg, vanilla extract, and buttermilk until smooth.

5-Slowly add the wet ingredients to the dry ingredients, whisking gently until smooth.

6-If using, whisk in the hot coffee or hot water slowly; the batter will be thin.

7-Pour the batter evenly into the prepared pan.

8-Bake for 27-30 minutes or until a toothpick inserted in the center comes out clean or with few moist crumbs.

9-Allow the cake to cool completely in the pan on a wire rack before removing.

10-Prepare ganache by heating heavy cream to a simmer and pouring over chopped chocolate; let stand 2-3 minutes before stirring to smooth. Cool and thicken before spreading or whip for frosting.

11-For buttercream, beat the butter until creamy, add powdered sugar, cocoa, milk, vanilla, and salt gradually, beating to fluffy consistency.

12-Spread or pipe the frosting or ganache onto the cooled cake and garnish as desired.