Cream margarine and brown sugar together with a wooden spoon or an electric mixer.

Add salt and vanilla extract, then the 3 egg yolks (keep your whites for later!). Mix until smooth in texture.

Add the flour slowly, mixing with your hands if needed. If the dough dosen't form itself after a throughout mixing, add a little bit of water and keep mixing.

One you have your ball of dough, cut 1/3 of it out and put it aside. Cut the rest into 6 equal parts (or the number of different food coloring you're going to use).

Punch a small hole into each dough balls (but the one you reserved at first) and pour a bit of food coloring into each of them. Roll the dough into your hands until the coloring is even. Put each different colors aside.

Roll each colored parts into equal almost perfect rectangles.

Cover each rectangle with the egg white you kept earlier. Put them all on top of the others and press them lightly.

Cut the sides with a very sharp knife to obtain a nice rainbow layered rectangle.

Cut that rectangle vertically into 4 equal pieces. Lay two pieces on their side and glue them together horizontally with the egg whites.

Take the reserved white dough and roll it flat. Cut it into four long rectangles and cover the sides of your rainbow dough layered rectangle with it.

Cover your dough roll with egg whites and roll it into your sprinkles.

Use a sharp knife and cut through your sprinkle covered cookie rectangle vertically into 1/2 inch thick pieces. You should obtain a clean, layered rainbow pattern. Lay the cookies flat onto a cookie pan covered with parchemin paper.

Take the rest of your rainbow dough and roll it flat. Take cookie cutters and make more cookies, decorate them, laugh and have fun!

Bake at 350 farahneit for 8-9 minutes and let cool completely.

Enjoy!