For the egg yolk base: Over a double boiler, gently whisk the egg yolks, elderflower cordial, filtered water and Cognac until it reaches 70°C (158°F). Once cooked, chill the base until completely cool.
To make the drink: Mix everything with a whisk until smooth. Adjust with more Cognac if needed. Strain the mix through a fine chinois. Keep cold and serve.