Bloom the gelatin by pouring 3 tablespoons of cool water into a small bowl and sprinkling the gelatin evenly over top. Allow the gelatin to sit until it has absorbed the water.
Place the white chocolate, sweetened condensed milk and white food color into a medium heat-proof bowl.
Combine the sugar, corn syrup and 1/4 cup of water in a small saucepan. Stir gently to ensure that the sugar is moistened. Heat the mixture over high heat until boiling and allow to boil for a minute or two until the syrup is clear. Remove the pot from the heat and stir in the bloomed gelatin. Pour the syrup and gelatin mixture into the bowl of chocolate and milk and allow to sit for 30 seconds before stirring until smooth.
Use an immersion blender to emulsify the glaze, making sure that the head of the blender stays below the surface of the glaze. This will ensure your glaze doesn't become aerated. Pour the glaze through a fine sieve if there are any lumps or air bubbles.
Color the glaze with powdered food colors by separating the desired amount of glaze into individual containers and stirring in the color. Mirror glaze should be poured when the temperature reaches 94 to 95 degrees F.