Preheat the oven to 350°F with a rack in the middle position. Line a metal 8" square pan with parchment or a reusable liner and lightly grease.  , To make the rhubarb jam: In a small pot, combine the rhubarb, sugar, water, and salt. Cook over medium-high heat until the mixture comes to a boil, about 2 minutes. Reduce the heat to medium-low and simmer until the rhubarb breaks down and the mixture is jammy. This can take anywhere between 5 and 15 minutes; it’ll take longer if your rhubarb is frozen.  , Remove the rhubarb jam from the heat and stir in the vanilla and lemon juice. Transfer the mixture to a small bowl (you should have a scant 3/4 cup, roughly 170g) and set aside. Do not wash the pot. , To make the bars: In a medium bowl, combine the brown sugar, salt, and vanilla. , In the same pot used to cook the rhubarb jam, melt the butter over medium-high heat, then pour it directly over the brown sugar mixture. , Whisk gently to combine, then vigorously for 30 seconds. The mixture will become thicker and slightly lighten in color. Add the eggs and continue whisking until just combined. , Using a flexible spatula, mix in the flour and baking powder. Before all the flour is fully incorporated, add the white chocolate and continue mixing until no streaks of flour remain. , Transfer the batter to the prepared pan and spread it evenly to the edges.  , Dollop heaping teaspoons of the rhubarb jam all over the top of the batter (a teaspoon cookie scoop works well here) then use a toothpick or a butter knife to swirl the jam into the batter in a decorative pattern. , Bake the rhubarb bars for 40 to 45 minutes until the center is set, the bars are browned, and a toothpick or thin knife inserted into the center comes out with a few moist crumbs. It’s OK if the toothpick or knife isn’t clean, but you don’t want to see any raw batter. , Remove the rhubarb bars from the oven and cool them completely in the pan before slicing and serving. , Storage information: Store any leftover rhubarb bars covered at room temperature for several days; freeze for longer storage.