Spray bottom and up sides of 9-inch springform pan with non-stick cooking spray. Set aside.
In a small saucepan over medium-high heat, whisk together sugar, water, and espresso coffee crystals and bring to a boil. Remove pan from heat and whisk in the chocolate chips until melted and smooth. Set aside to cool slightly.
To a small bowl, add egg yolks and vanilla extract and whisk to combine. Temper the eggs (See Note 1) by whisking the yolks into the chocolate mixture in small increments until thoroughly blended. Set aside.
To a medium bowl, add flour, cocoa powder, cinnamon and baking powder and whisk to combine. Slowly whisk in the egg yolk and chocolate mixture until smooth.
To large mixing bowl or bowl of stand mixer, add egg whites. Use electric hand mixer or stand mixer on high to beat egg whites until stiff peaks form. With a spatula, gently fold the chocolate mixture into the egg whites, folding over to incorporate until all are combined and the mixture is light. Do not over mix!
Spread cake batter into prepared 9-inch springform pan.
Bake for 30 minutes or until the top springs back to the touch. Leave cake in pan and place onto wire rack to cool completely before frosting.
Frosting
While cake is cooling, beat together the softened butter, powdered sugar, cocoa and milk in mixing bowl to form a thick, smooth frosting. Alternatively, use a stand mixer fitted with the paddle attachment.If the frosting is too thick for your liking, incorporate 1 tsp. of milk at a time until you reach your desired consistency.
When the cake has cooled, remove from springform pan, transfer to a cake plate and spread frosting evenly over the top of the cake, making sure not to drag crumbs. Top with berries or shaved chocolate if desired (as pictured).
Notes
To prevent the eggs from cooking, temper them with the chocolate mixture. Add a couple of tablespoons of the hot melted chocolate and whisk it slowly into the egg yolks. Keep adding in small increments, until the egg yolks are no longer cold. Once about half of the chocolate mixture is combined with the yolks, you can combine them without any issues.