Preheat an oven. Spray a cake pan with baking spray, then line the bottom with a round of parchment paper.
Place chocolate chips and unsalted butter in a large microwave-safe bowl, then heat on high, stirring after each addition, until both butter and chocolate are completely melted.
Sift sugar, Kosher salt, and espresso powder in with the chocolate and butter mixture, then add vanilla extract and whisk to combine. Add eggs, one at a time, and beat until smooth. Add sifted cocoa powder, then whisk until just combined.
Pour the batter into the greased cake pan, use an offset spatula to smooth the top into an even layer, then bake until a thin crust forms on top of the cake. Cool, then run a knife along the edge of the pan to loosen. Invert the cake onto a serving plate or cooling rack.
Allow the cake to cool. Serve at room temperature, cool completely, or reheat until warm. Enjoy with fresh berries and/or whipped cream!