Preheat your oven to 325°F (165°C). Line an 8x8-inch baking pan with parchment paper, leaving a little overhang so you can lift the cookies out easily later.
Cream the butter and sugar. In a large mixing bowl, beat the softened butter and powdered sugar together until smooth and creamy.
Mix in the vanilla extract.
Stir in the flour and salt until a soft dough forms. Be careful not to overmix, as this keeps your shortbread tender and buttery
Fold in most of the red and green M&Ms, saving a small handful for the top
Press the dough evenly into the prepared pan. You’ll have to press a bit hard, as the dough is kind of flaky. Then, chill your dough in the pan in the fridge for about 30 minutes.
After chilling, carefully lift your shortbread out of the pan and cut it into 1-inch squares.
You can bake again in the 8x8 pan if you wish, but I chose to lay mine out on a lined baking sheet. Bake for 25–35 minutes, or until the edges are just turning golden brown. For brighter pops of color, press a few more M&Ms on top around the 15-minute mark while the cookies are still baking.
If you used an 8x8 pan, let the pan cool completely, then lift the shortbread out by the parchment and cut or break apart along the cut lines you already made. If you baked on a baking sheet, just let the little bites of joy cool right there on the baking sheet.
Optional: Dust with powdered sugar or drizzle with melted white chocolate for an extra-festive touch.