Line a cookie sheet with parchment paper or grease it well.
Cream the softened butter & sugar together until smooth with a hand mixer and then add in the egg (or vegetable oil) and the vanilla.
Mix the flour, baking powder, and salt in a separate bowl.
Slowly add the dry ingredients into the butter mixture. Fold in until well combined.
Add a couple drops of green food coloring if you like. This step is optional, but you can't go wrong with a green St Patrick's Day Cookie! Be aware that your hands might be a little stained if you touch the uncooked dough.
Turn out the dough onto a floured surface and roll to about 1/4 inch thick. Add flour as needed to combat stickiness.
Use a metal Shamrock Cookie Cutter to shape your St. Patrick's Day cookies.
Sprinkle the top of each cookie with your green sprinkles or colored sugar.
Quickly slide a spatula under the cookies and transfer them to your baking sheet. Bake for 8-10 minutes at 350 degrees F. Don't wait for them to brown or they will be too hard.
Let the Shamrock Sugar Cookies cool and then enjoy your sweet St Patrick's Day Cookies!