Make shells by mixing the butter, cream cheese and flour until dough is formed. (You can either mix by hand or use a food processer). Roll dough into balls, approximately 1", and press into ungreased mini muffin tins. Be sure to bring dough up sides of each muffin cup or filling will stick to pan.
Make filling by mixing the eggs, brown sugar, vanilla, and walnuts. Place 1 Tbs. of filling into each muffin cup.
Bake at 375F for 30 minutes or until golden brown.
Cool in pan 10 minutes, then remove tarts and cool completely on wire rack. Dust with powered sugar before serving.