Step 1:
In a small saucepan, combine rhubarb, sugar, brown sugar, and water. Bring to a boil over medium heat. Reduce heat and simmer, stirring occasionally, for about 10 minutes, until the mixture thickens and the rhubarb softens. Let cool.
Step 2:
In a saucepan, heat cream over medium heat until almost simmering. In a separate bowl, stir together egg yolks, whole eggs, and granulated sugar just until blended. Gradually stir the hot cream into the egg mixture. Strain the mixture through a fine sieve into a clean bowl and stir in vanilla.
Step 3:
Divide the cooled rhubarb purée evenly among six 6 oz (175 mL) ramekins or custard cups. Carefully spoon the custard mixture over the rhubarb.
Place the ramekins in a large roasting pan. Pour enough hot tap water into the pan to come halfway up the sides of the ramekins. Bake in a 300°F (150°C) oven for about 1 hour, or until the custard is set but still quivers slightly when gently shaken.
Step 4:
Carefully remove the ramekins from the water bath and let them cool to room temperature on a wire rack. Cover each ramekin tightly with plastic wrap and refrigerate for at least 6 hours or up to 2 days.
Step 5:
When ready to serve, gently blot any liquid from the surface of the custards with paper towels. Sprinkle 2 tsp (10 mL) of brown sugar evenly over the top of each custard. Place the ramekins on a baking sheet and broil as close to the heat source as possible until the sugar melts and caramelizes, about 2 minutes. Let stand until the sugar hardens.
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