Preheat oven to 350°F (175°C). Grease and line two 8" cake pans, a bundt pan, tube pan, or one 9x13 prepared pan with pan spray and parchment.
In a large mixing bowl, whisk together flour, cocoa powder, espresso powder, baking soda, and salt. Set aside.
In another large bowl (or the bowl of a stand mixer fitted with paddle attachment), cream butter and sugar on medium speed until light and fluffy, about 3–4 minutes. Scrape the sides of the bowl as needed.
Beat in egg yolks one at a time on low speed. Add vanilla, sourdough discard, and Greek yogurt (or sour cream). Mix until smooth.
In a separate clean bowl, whip egg whites with an electric mixer until soft peaks form. Fold gently into the batter—don’t stir, or you’ll deflate the egg whites.
Gradually add dry ingredients into the wet ingredients, stirring gently until just combined. Add a splash of brewed coffee or milk if the batter feels too thick.
Pour into prepared pan, smooth the top, and bake for 30–35 minutes, or until a long toothpick inserted in the center comes out with a few moist crumbs.
Place on a wire rack to cool completely before glazing.
For the ganache: In a small saucepan over medium heat, warm the cream until steaming. Pour over chocolate chips in a heatproof bowl, let sit 2–3 minutes, then stir until smooth. Add vanilla and sea salt. Drizzle over the top of the cake.