For the crust, in a medium bowl, combine the flour, sugar, salt and vanilla extract. Sprinkle the butter pieces over the top of the dry mixture and using a pastry blender, two knives or your fingers, cut the butter into the dry mixture until it resembles coarse crumbs.
Pour the mixture into the bottom of a 9X9-inch pan and press the crust firmly to form an even layer. B
Bake the crust for 16-17 minutes, until lightly browned around the edges.
While the crust is baking, prepare the filling by whisking together all of the filling ingredients in a large bowl until evenly combined.
Pour the filling mixture over the hot crust when it comes out of the oven and return it to the oven to bake for 18-20 more minutes, until the filling is set (it will not jiggle if the pan is shaken slightly).
Cool the bars in the pan completely before dusting with powdered sugar, slicing and serving. These bars also taste excellent after they have been chilled.
Buttermilk: here is a guide to making your own buttermilk.