Preheat oven 375° F. Line the bottom of a 9-inch springform pan with parchment paper and grease with nonstick cooking spray (the sides of the pan too!) Set aside.¹
In a large microwave safe bowl, melt the butter and chocolate chips in the microwave for 45 seconds. Stir the mixture until the chocolate chips are completely melted. If necessary, place bowl back in the microwave for another 10 seconds.²
Add sugar, vanilla and salt. Stir with a rubber spatula or wooden spoon until combined, about 20 seconds. Add eggs and stir until incorporated, about 1 minute.
Carefully add the cocoa powder and espresso powder (if desired). Stir just until combined, careful not to overmix.
Pour batter into prepared pan and bake for 25 minutes. Cool on a wire rack for 10 minutes. Run a butter knife around the edges of the pans and turn cake upside down on a plate. The bottom of the cake will be nice and smooth, and is now the top of the cake. Serve warm or at room temp with fresh fruit, powdered sugar or ice cream.
Melting chocolate: Feel free to continue microwaving the chocolate in 10 second increments until melted. Stir after every 10 seconds. See more tips for melting chocolate.How to store flourless chocolate cake: Cover with plastic wrap or reusable food wrap and store in the fridge for up to one week. You can also freeze flourless chocolate cake for up to 3 months if it is tightly wrapped.Recommended tools