Hey there, welcome to the first of perhaps many coffee soda recipes.
The idea here is that you can make a batch of these, rather than just one. What I’d recommend doing is scaling this recipe, depending on how much paper filtered espresso you have or want to make. I’m using 50g of it here to make the scaling side a little bit easier.
50g paper filtered espresso (From a 1:2.2 ratio espresso, or espresso with approximately 9-10% strength)
235g filtered water
35g simple syrup (2 parts sugar to 1 part water)
1g Fee Brothers' Aztec Chocolate Bitters
0.7g saline (20:80 salt to water ratio)
Mix well and refrigerate before carbonation. Charge with CO2 and leave for 1 hour. Open carefully. Pour gently. Enjoy.
Note: If you want to use a Sodastream, then combine all ingredients except water and then chill. Carbonate clean water and then to serve add the required weight to your coffee syrup. (Sodastream don’t like it when you put things other than water in there, so may not honour warranties if you were to have issues.)