For the butter crust: Place the flour, sugar and salt into the well of a food processor and pulse a few times. Add the cold butter cubes and pulse until the mixture is fine and mealy. Add the egg and pulse, then run the food processor until the dough comes together.
Turn the dough out onto an 8-inch square tart pan and evenly press into the bottom and sides of the pan. Using a fork, prick the bottom of the crust all over to prevent bubbling. Par-bake for 12 to 15 minutes or until the crust has barely cooked through and is still pale. Remove from the oven and cool completely.
For the rhubarb: Trim both ends of each rhubarb stalk so it is 8 inches long. Slice each stalk in half lengthwise, keeping the sliced halves together. Slice each halved stalk into eight 1-inch squares. Slice each square in half to create triangles (128 pieces total).
For the almond filling: Place the butter and sugar into a mixing bowl and beat together using an electric hand mixer until light and fluffy. Add the eggs and continue to mix together. Beat the almond meal, flour and vanilla into the butter mixture until fully incorporated. Pour the filling into the cooled crust and spread evenly using a spatula.
To assemble: Starting from the top left corner, take 4 triangles (top pieces) and arrange the pieces together to create a "frame" with a diamond pattern in the center. Repeat until all top triangle pieces have been used, forming 16 "frames." Take the bottom triangle pieces and arrange them inside each "frame" cut-side up, creating a diamond shape. Repeat until all of the rhubarb pieces have been used and the tart is filled.
Brush the top of the tart with a little bit of water and sprinkle with the sugar. Bake the tart for 25 to 30 minutes, until the filling has just cooked through, the top is bubbling and the crust is browned. Allow the tart to cool, about 1 hour. Remove the tart from the pan, slice and serve.