Wash the rhubarb stalks, cut off the ends, and chop them into small pieces less than 1/2-inch wide. You should have about 2 cups of chopped rhubarb.
Combine the rhubarb, raspberries, sugar, vanilla, and lemon juice in a medium saucepan over medium heat. Stir occasionally while it heats up and the sugar melts, then once it comes to a simmer, lower the heat to medium-low and continue to cook the jam until the fruit breaks down, most of the liquid boils off, and it has a thick, jammy texture. (It will continue to thicken as it cools.) This should probably take about 15-20 minutes.
Once the jam is cooked, remove it from the heat and cool it to room temperature before using. It can be made in advance and kept in the refrigerator for up to 2 weeks.
To Make the Almond Crust and the Bars:
Preheat the oven to 350 F. Line a 9x13-inch baking pan with foil and spray the foil with nonstick cooking spray.
Coarsely chop the almond paste into pieces and put it in a food processor. Process in long pulses until the almond paste is in fine crumbs, then remove it from the processor.
Place the flour, powdered sugar, salt, and baking powder in the processor, and pulse briefly to combine. Add the cold cubed butter, and pulse in long pulses until the mixture is crumbly, but holds together if you press a ball of it between your fingers. Add the almond paste crumbs and almond extract, and process in short bursts until it's well-mixed. (The dough will still be crumbly.)
Separate out about 1 1/4 cups of the crust and set it aside for later. Take the remaining crust mixture and press it into an even layer in the bottom of the pan. Bake the bottom crust at 350 F for 15 minutes, until it is puffed and just starting to take on some color.
Spread the jam out into an even layer on the baked crust, and sprinkle the reserved crust mixture over the top. (Don't worry about getting perfectly even coverage, it will puff and spread as it bakes.)
Bake the bars for an additional 28-43 minutes, until the filling is bubbling and the top crust is golden brown. Remove from the oven and cool to room temperature.
Using the foil as handles, remove the bars from the pan and cut them into small squares. Top them with a dusting of powdered sugar. Remaining bars can be kept in an airtight container in the refrigerator for up to a week.