Rhubarb-Mascarpone Mousse Cake

INGREDIENTS

• 1/2 cup water, divided
• a few small strawberries, such as tri-star or wild strawberries with their leaves (optional)
• 3 tablespoons water
• 2 pounds rhubarb, cut into 1-inch pieces (8 cups)
• 1 large egg, at room temperature 30 minutes
• 1 teaspoon vanilla
• 1 cup rhubarb syrup (see below)
• 1/2 cup sugar
• 1/4 teaspoon salt
• Vegetable oil for greasing pan
• 1 1/2 teaspoons unflavored gelatin
• 1/2 stick (4 tablespoons) unsalted butter, softened
• 1 cup (8 ounces) mascarpone
• 3/4 cup chilled heavy cream
• 3 teaspoons unflavored gelatin (less than 2 packages)
• 1 1/2 cups sugar, divided
• 1/3 cup whole milk
• 1 teaspoon baking powder
• 1/2 cup strawberry or strawberry-rhubarb jam
• 1 cup all purpose flour