📖 Instructions

1. **Before you begin:** Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy lifting later. If you don't have parchment, lightly grease and flour the pan, but parchment really is a lifesaver here.
2. **Cream the butter and sugar:** In a large mixing bowl, or the bowl of a stand mixer fitted with the paddle attachment, cream together the softened unsalted butter and granulated sugar on medium speed until the mixture is light, fluffy, and noticeably pale in color. This usually takes about 2-3 minutes. Don't rush this step, as it incorporates air, which contributes to the cookies' texture.
3. **Add the egg and vanilla:** Beat in the large egg until it's just combined. Scrape down the sides of the bowl with a rubber spatula to make sure everything is incorporated. Then, add the pure vanilla extract and mix until fragrant, about 30 seconds.
4. **Combine dry ingredients:** In a separate, medium bowl, whisk together the all-purpose flour, baking powder, and salt. Make sure there are no lumps and the ingredients are evenly distributed.
5. **Gradually add dry to wet:** With the mixer on low speed, slowly add the dry ingredient mixture to the wet ingredients, a little at a time. Mix only until the flour streaks have almost disappeared. Be careful not to overmix the dough, as this can lead to tough cookies (and nobody wants a tough cookie!). The dough will be thick.
6. **Press into the pan:** Using your rubber spatula or clean, slightly floured hands, evenly press the thick cookie dough into the prepared 9x13 inch baking pan. Try to get it as level as possible from edge to edge. A little trick: lightly dampen your fingertips with water to prevent sticking.
7. **Bake the bars:** Place the pan in your preheated oven and bake for 20-25 minutes, or until the edges are lightly golden brown and the center looks set and no longer glossy. The middle might still look *just* slightly underbaked, but it will firm up as it cools. Overbaking is the enemy of a soft cookie bar!
8. **Cool completely:** Remove the pan from the oven and place it on a wire rack to cool completely. This is super important! The bars need to be entirely cool before you even think about frosting them, or your beautiful frosting will melt into a sad, sugary puddle. This can take at least 1-2 hours.
9. **Prepare the frosting:** While the bars are cooling, make your frosting. In a medium mixing bowl (or the bowl of your stand mixer), beat the softened unsalted butter on medium speed until creamy, about 1-2 minutes. Gradually add the powdered sugar, one cup at a time, mixing on low speed until combined, then increase to medium and beat until smooth. If your mixer is on too high a speed from the start, you'll end up with a cloud of powdered sugar! Oops, been there, done that.
10. **Add milk, vanilla, and color:** Add the whole milk and vanilla extract to the frosting. Beat on medium speed until the frosting is light and fluffy. Now for the fun part: divide your frosting into a few small bowls if you want different colors. Add 1-2 drops of gel food coloring to each bowl and mix until you achieve your desired pastel shades (think light pink, sky blue, soft yellow, mint green).
11. **Frost the bars:** Once the cookie bars are completely cool, use an offset spatula or the back of a butter knife to spread an even layer of your colored frosting(s) over the top. If using multiple colors, you can swirl them together for a pretty marbled effect, or pipe distinct sections.
12. **Decorate and serve:** Immediately after frosting, scatter your favorite Easter sprinkles over the top. The frosting needs to be fresh for the sprinkles to stick well. Once decorated, use the parchment paper overhang to lift the entire slab of cookie bars out of the pan. Place on a cutting board and cut into 16 (or more!) squares. Enjoy your easy, festive treats!