Set up your water bath and immersion circulator according to your equipment. Heat water bath to 198°F (92.2°C) and set cook time for 1 hour. Lightly coat a 6-inch round cake pan with non-stick spray. Line bottom of pan with parchment paper and spray with no-stick spray.
In medium bowl, whisk flour, cocoa powder, baking soda and salt; set aside.
In another bowl, whisk brown sugar and egg, about 30 seconds, making sure there are no lumps. Add coffee, buttermilk, butter and vanilla; whisk until well blended.
Add flour mixture; gently whisk until dry ingredients are just moistened.
Place magnet inside 1- gallon Ziploc bag. Insert pan into bag centering pan over bottom of bag and magnet. Cover pan with a heat proof plate. Seal bag pushing out as much air as possible. Attach bag magnet to a weighted magnetic metal pan that will fit inside your water bath container. Drop sealed cake into water bath making sure cake pan is submerged to its rim or slightly below water. Make slight adjustments, if needed, to be sure cake pan is level. (As cooking proceeds, bag may balloon; be ready to expel additional air.)
Cook cake for 1 hour. Remove from water bath and open bag to let heat escape. Carefully remove cake from bag and remove plate. Let cake cool in pan for 10 minutes.
Run a thin knife around the edge and invert on to cooling rack. Remove parchment paper. Cool completely.
This cake is delicious on its own served with whipped cream or a dusting of powdered sugar, but I like slicing it in half, horizontally, and turning it into a layer cake. I filled and frosted this one with a mix of equal parts Nutella and mascarpone cream (about 4 ounces of each) and then topped with fresh raspberries.