In a small heavy bottomed pot add the blueberries, sugar, instant espresso, and salt.
Mash the blueberries with a muddle and place the pot over medium heat.
Heat the mixture until it comes to a boil and cook until all the sugar is dissolved for about 5 minutes.
Strain the mixture through a 250 micron super bag and allow the syrup to cool before use.
In a small heavy bottom saucepan melt the butter and heat the butter until the milk solids begin to brown, reduce the heat to medium low add the milk powder. Cook until golden brown and crispy.
Drain the milk solids on paper towels and reserve the brown butter for rimming the glass.
Mix the brown butter solids with brown sugar and place it on a small plate wide enough for the glass to be rimmed.
In a whipping siphon add the blueberry syrup, water, bitters, and aromatic elixirs. Screw on the top and charge with 1 CO2 cartridge. Shake thoroughly.
Release the pressure and charge a second time. This will make sure the soda holds its carbonation. Shake once more.
Gently dip the rim of a rocks glass in a very thin layer of browned butter and rim the glass with browned butter solids.
Add ice to the glass. Release the pressure from the whipping siphon and pour the contents over the ice.
Top the drink off with elderflower tonic water and serve.