Preheat oven to 350°F (177°C). Grease a 6-inch round cake pan, line with a parchment paper round, then grease the parchment paper. Parchment paper helps the cake seamlessly release from the pan. (If it’s helpful, see this parchment paper rounds for cakes video & post.)

Whisk the flour, cocoa powder, baking soda, espresso powder, and salt together in a large bowl. In a medium bowl, whisk the oil, sugar, egg, vanilla extract, and buttermilk together in a medium bowl. Pour the wet ingredients into the flour mixture and whisk until smooth and combined.

Pour the cake batter evenly into prepared cake pan.

Bake for around 27–30 minutes or until the cake is baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it’s done. Avoid over-baking. Cake is thick and moist, so the center may slightly sink. That’s normal. Allow cake to cool in the pan set on a wire cooling rack. Cool completely before frosting.

Make the chocolate ganache as the cake cools because the ganache must cool too: Place chopped chocolate in a medium heat-proof bowl. Heat the cream in a small saucepan over medium heat until it begins to gently simmer. (Do not let it come to a rapid boil—that’s too hot!) Pour over chocolate, then let it sit for 2–3 minutes to gently soften the chocolate. With a metal spoon or small silicone spatula, very slowly stir until completely combined and chocolate has melted. Ganache will be thin, so let it cool and thicken for at least 30–60 minutes before using. Refrigerating speeds this up, but the ganache will not cool evenly. Stir it a few times as it sets in the refrigerator so it remains smooth. You can also let the ganache fully cool and thicken (takes about 2 hours). After 2 hours, you can whip the cooled ganache with a handheld or stand mixer fitted with a whisk attachment until light in color and texture, about 3–4 minutes on medium-high speed. (Whipped ganache can be spread or piped. It is pictured on the cupcakes in the post above. I used Wilton 1M piping tip.)

Spread ganache on cooled cake. If ganache has cooled and thickened enough (or if you whipped it), you could pipe it with a piping bag/tip. Decorate with optional garnishes, if desired. Slice and serve.

Cover leftover cake tightly and store at room temperature for 1 day or in the refrigerator for up to 5 days.