Heat milk and 1/2 cup of the sugar in a medium saucepan over medium heat, stirring until sugar dissolves. Heat gently until milk just starts to bubble around the edges, do not boil. Pour into a heatproof bowl and stir in hibiscus flowers. Let cool then cover and refrigerate overnight.
Heat oven to 325°F. Place a 9-inch ring mold or 8 (4-ounce) ramekins in a roasting pan. Bring a large kettle of water to a boil.
Combine 1 cup of the sugar and the 1/4 cup of water in a small saucepan over medium-high heat. Stir to dissolve sugar then let cook, undisturbed, until amber in color, about 8 minutes. Immediately pour the hot caramel into the bottom of the ring mold, or divide it evenly between the ramekins, swirling to coat.
Whisk eggs, yolks, remaining 1/2 cup of sugar, and salt in a large bowl.
Strain the hibiscus-milk mixture through a fine mesh sieve into a medium saucepan. Place over medium heat just until little bubbles form around the edges of the pan.
Slowly add warm milk into the egg mixture, whisking continuously until smooth. Strain mixture one last time then pour into mold or ramekins.
Place roasting pan in the oven then add boiling water, making sure it reaches halfway up the sides of the mold or ramekins. Bake until set, about 30-35 minutes.
Remove to a wire rack and let cool 30 minutes. Cover and chill in the refrigerator for at least 8 hours.
To serve, run a sharp knife around the inside edge or the mold or ramekins, place a plate on top, invert and lift mold off. If flan is not releasing briefly dip the bottom of the mold or ramekins in hot water and try again.
