Please note that this recipe was very possibly written by my grandmother and doesn't have instructions like you might find in a cookbook, such as the order in which to mix things together. Good luck!
1 c softened butter
3/4 c powdered sugar
2 c flour
1 tsp vanilla*
pinch of salt
Mix all ingredients together. Chill a short time so dough is easier to work with. Roll out on cutting board, cut into rectangles and add lines with butter paddle.** Place on cookie sheet and bake 8-10 minutes at 350° F until edges are slightly brown. Remove from pan and let cool on cooling racks. When completely cool microwave a small amount of chocolate chips and mix with a little milk until smooth. Dip end of cookie into chocolate and then dip into sprinkles or coconut.*** Let chocolate harden before storing in cookie tin.
Here's a picture I found of some finished Mabel's Favorites from a couple years ago.
*I always double the vanilla, but maybe my blatant disregard for following directions is part of the trouble I'm having with the dough. So do with that information what you will.
**I had no idea what this thing was called until I did some googling while writing this out. We've always used the ridged part to add vertical lines on the rectangles just to make them more fancy, though that of course is purely voluntary.
***Do not dip into coconut. Coconut is disgusting.