Pour the water into a small stainless-steel saucepan. Sprinkle the gelatin evenly over the surface and leave it for 1 minute to soften.
Set the pan over medium heat and stir until the gelatin dissolves. When the water is steaming but not simmering, turn off the heat.
Stir in the raw honey until dissolved. Add the cinnamon if using it, plus the salt. Cool the mixture completely to room temperature.
Put the softened butter in a large stainless-steel bowl and beat with an electric hand mixer until creamy and free of firm lumps.
Add the cooled honey-gelatin mixture in a slow stream while beating. Continue until smooth, glossy, and evenly incorporated. If using homemade vanilla instead of cinnamon, beat it in now.
Spread or pipe over a completely cooled cake while the frosting is at room temperature.