In a large bowl, cream butter and sugars until light and
fluffy. Add eggs, one at a time, beating well after each
addition. Combine the flour, baking soda and salt; gradually
add to creamed mixture and mix well (dough will be sticky).
For filling, combine the rhubarb, sugar and 2 tablespoons
water in a large saucepan. Bring to a boil. Reduce heat;
simmer, uncovered, for 10 minutes or until thickened,
stirring frequently. Combine cornstarch and remaining
water until smooth; stir into rhubarb mixture. Bring to a
boil; cook and stir for 2 minutes or until thickened.
Remove from the heat; stir in vanilla.
Drop dough by tablespoonfuls 2 in. apart onto ungreased
baking sheets. Using the end of a wooden spoon handle,
make an indentation in the center of each cookie; fill
with a rounded teaspoon of filling. Top with 1/2 teaspoon
of dough, allowing some filling to show. Bake at 375°
for 8-10 minutes or until lightly browned.