Preheat the oven to 325°.
Put the diced rhubarb, maple syrup and diced rosemary in a small pan and cook over medium heat, stirring occasionally, until the rhubarb is soft. Remove from heat and let cool.
While the rhubarb is cooking, put the cream, vanilla and rosemary sprigs into a small pan and, stirring occasionally, cook over medium-high heat until the cream just starts to bubble. In a medium bowl, beat the egg, egg yolk and maple syrup with a whisk until even. While whisking, slowly pour the hot cream into the egg mixture. Strain the egg and cream mixture into a large measuring cup with a spout (capacity at least 2 cups).
Divide the rhubarb mixture among the ramekins. Place the ramekins in a casserole dish and place the whole dish on the middle rack of the preheated oven. Pour the cream and egg mixture into each ramekin, over the rhubarb mixture (it should just fill the ramekin). Then pour enough hot tap water into the casserole dish that the water reaches halfway up the side of the ramekins. Bake the custards in the water bath for about 35 minutes or until the custards still quivers when shaken but have no liquid beneath the skin. Cool the custards in the fridge for 1 hour, or up to three days.
When the custards are cool, turn on your oven broiler, evenly spread 1 teaspoon of maple sugar over each custard and place the custards on a cookie sheet. Move the oven rack to the highest location in the oven and put the sheet of custards under the broiler. The sugar will take about 5 minutes to burn, although broilers vary widely in intensity, so keep a careful eye on the crème brûlées. The crème brûlée is done when the maple sugar is a mix of light brown and black. Quick-cool the crème brûlées in the fridge for about 1 minute and then serve immediately.