In a bowl, cream together butter, sugar, vanilla extract, and almond extract until light and fluffy. Scrape down sides of bowl.
In a separate bowl, sift (or whisk) together the flour and salt. Add dry ingredients to butter mixture and mix until a dough forms (dough may be slightly crumbly, but holds together when pinched together). Add rainbow jimmies and mix until just combined.
Press dough into a parchment lined 9x13 baking pan and refrigerate for 30 minutes (chilling the dough helps prevent unwanted spreading in the oven). Meanwhile, preheat oven to 350 degrees Fahrenheit and line a baking sheet with parchment paper. Set aside.
Pull dough out of the pan with the parchment paper. Slice dough into about ¾" rows across and down to create tiny squares. Place squares about 1 inch apart on parchment lined baking sheet. It will take 2 or more cookie sheets to bake all of your cookies, so store any remaining dough in the refrigerator until ready to bake.
Bake for 8-10 minutes, or until cookies begin to lightly brown around the edges. Remove from oven and allow to cool for 2-3 minutes before removing from sheet to cool completely.
Confectioner's sugar can be substituted with granulated sugar. Substitute ¾ cup of confectioner's sugar with ½ cup of granulated sugar. My original recipe used granulated sugar, but I find confectioner's sugar easier to work with in shortbread dough.
Cookies will keep for 5-7 days in a sealed container. To help keep cookies fresh, place a slice of bread in the container with the cookies. Replace the bread slice as needed.
Raw dough will keep in the refrigerator for 3 days. If dough has been refrigerated for more than 30 minutes, allow to warm up at room temperature for 10-15 minutes before rolling out.
Cookie dough and baked cookies freeze well, up to 3 months in a sealed plastic container or freezer bag.
Shortbread cookies are very tender straight from the oven. Allow them to cool before handling.