Preheat oven to 300 degrees F. Line the bottom of a 9-inch spring form pan with parchment paper. Spray the inside of the pan and top of the parchment paper with cooking spray. Wrap the bottom of the spring form pan with heavy duty aluminum foil. Separately, fill a large casserole dish (I used 11x13 inch) or baking sheet with about 1/2 inch of water to form a water bath.
In a small saucepan over medium heat, combine sugar, coffee and salt. Stir well and heat just until the sugar and salt have fully dissolved. Remove from heat.
Place chocolate in microwave safe bowl. Heat in 1-minute increments on 50% power to melt, stirring well in between each minute. Alternatively, you can melt chocolate in a double boiler. As soon as all the chocolate has melted, transfer to the bowl of a stand mixer.
Using the paddle attachment, beat on low to medium speed, adding one tablespoon of butter at a time until all of the butter has been added and it has melted into the chocolate. With the mixer running, add the coffee sugar mixture and blend until fully mixed. Add the vanilla and Grand Marnier; mix well to combine and scrape sides of bowl as needed.
With the mixer still running, slowly add eggs. You want them fully incorporated, but you do not want to overly mix the batter. Scrape the sides of the bowl well and give a final stir to make sure everything is fully mixed. Transfer all of the batter to the prepared spring form pan.
Place the aluminum foil lined pan in the water bath and bake in preheated oven for 45-50 minutes. The center of the cake should be set but slightly jiggly.
For best results, chill cake in pan over night. I was able to skip this step and remove it from the pan a couple hours after I took it out of the oven, but it was even better after a night of refrigeration. To remove from pan, release spring to life side off and transfer to a cake platter.
Serve with freshly whipped cream or a scoop of vanilla ice cream and fresh berries.