Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9 inch pie pan.
In a medium bowl, cream together the butter and 1/2 cup white sugar until light and fluffy. Stir in the flour, baking powder and salt. Gradually stir in the milk until the dough is soft and pliable. Press the dough into the prepared pie pan.
In a medium saucepan, combine the plums, 1/2 cup white sugar, cornstarch and water. Cook over medium heat until thick. Pour the filling into the crust.
Bake for 30 minutes in the preheated oven, or until the crust is golden brown. Cool before topping with a glaze.
To make the glaze, combine the confectioners' sugar, butter and milk in a small saucepan. Cook over low heat until the mixture is thick and smooth. Drizzle over the cooled tart.
Interesting Facts
Plums are a member of the Prunus genus, which also includes peaches, nectarines and cherries.
Plums have been enjoyed as a fruit for thousands of years and have been found in archaeological sites throughout Europe.
Plums come in a variety of colors, including red, yellow, green and purple.