Mix softened butter with sugar, then mix in the egg.
Add the flour mixture and mix until a ball is formed, try not to overmix.
Flatten the pastry into a large round Tupperware (approximately 12”), put the cover on and refrigerate for approximately 20 minutes, or until it is firm (If you don’t have a large Tupperware, you can flatten it onto a large plate and cover with plastic wrap). When the dough is ready, roll it out onto a lightly floured surface into the shape of your tart pan (I use a 9 inch round tart pan with a removable bottom) approximately 1/8 inch thick and 1 to 2 inches wider than your pan (depending on how tall it is).
Grease the tart pan (I use butter, but please use whatever you prefer) and lightly flour it. Lay the rolled out dough into the pan and gently press it into the bottom and on the sides of the pan. Prick the dough 6 - 8 times with a fork. Cover it with plastic wrap and refrigerate it for 20 more minutes. Preheat your over to 400 degrees F. When ready, take the tart pan out of the refrigerator and fill it with pie weights (I used dried beans). Bake for 22 minutes (crust should be golden brown). Remove weights and cool the crust.
Mix cornstarch and sugar together in saucepan, then add water and mix again. Add rhubarb to mixture and cook on medium high, stir every minute or so until the rhubarb is very soft (if your rhubarb is stringy, you will need to cook it longer). You want the mixture to be at a slow boil to cook the rhubarb. Once it is nice and soft, take off the stove and add in the vanilla and raspberry liqueur. Wait for it to cool.
Remove the tart from pan. Spread the rhubarb mixture evenly in the tart. Can eat warm or chilled in the refrigerator. Some people like whipped cream or vanilla ice cream on top. I like it the way it is.
Cut it up, serve it out and enjoy.