📖 Instructions

1. Prep Your Pan and Oven: First things first, get your oven preheating to a cozy 350°F (175°C). Then, line a 9x13 inch baking pan with parchment paper, leaving a little overhang on the sides. This is your secret weapon for easy lifting and less messy clean-up later, trust me. Lightly grease the exposed sides of the pan too, just for good measure.
2. Cream the Butter and Sugar: In a large mixing bowl, either with your trusty electric mixer (stand or handheld works wonders here) or some serious arm power and a whisk, cream together the softened unsalted butter and granulated sugar. You're aiming for a light, fluffy, and pale yellow mixture. This usually takes about 2-3 minutes of mixing. Don't rush this step; it incorporates air, which gives your bars that tender texture.
3. Add the Egg and Vanilla: Crack in the large egg and pour in that glorious vanilla extract. Continue to beat until everything is well combined and smooth. Sometimes it might look a little curdled at first, but keep mixing, and it'll come together beautifully.
4. Combine Dry Ingredients: In a separate medium-sized bowl, whisk together the all-purpose flour, cornstarch, baking powder, and salt. The cornstarch is a small but mighty addition here; it helps make the bars extra tender and melt-in-your-mouth.
5. Mix Wet and Dry: Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until *just* combined. Stop as soon as you no longer see streaks of flour. Overmixing develops the gluten too much, which can lead to tough bars, and we want tender, remember?
6. Press into Pan: Take that lovely dough and press it evenly into your prepared 9x13 inch baking pan. A rubber spatula or the back of a measuring cup works great for this. Make sure it's nice and smooth across the top for even baking.
7. Bake the Cookie Base: Pop the pan into your preheated oven and bake for 20-25 minutes. The edges should be lightly golden, and the center should be set and appear firm when you gently touch it. A little underbaked is better than overbaked for soft bars!
8. Cool Completely: Once baked, remove the pan from the oven and place it on a wire cooling rack. Let the cookie base cool completely in the pan. This is super important! Trying to frost a warm cookie base will result in a melty mess, and no one wants that. Patience, my friend.
9. Prepare the Glaze: While the bars are cooling, let's get that "snowy edge" glaze ready. In a small saucepan over low heat, gently melt the unsalted butter. Once melted, remove it from the heat.
10. Whisk the Glaze Ingredients: Sieve the powdered sugar into a medium bowl. This step is a must to avoid lumps in your glaze – nobody likes lumpy snow! Add the melted butter, milk (start with 2 tablespoons, you might need a tiny bit more), and vanilla extract to the powdered sugar. Whisk vigorously until smooth and creamy. The consistency should be pourable but still thick enough to coat the back of a spoon. If it's too thick, add a *tiny* splash more milk; if too thin, add a *tiny* spoonful of powdered sugar.
11. Glaze the Bars: Once the cookie base is completely cool, carefully lift it out of the pan using the parchment paper overhang and place it on your wire cooling rack (with something underneath to catch drips!). Pour the vanilla glaze over the cooled cookie bars.
12. Create Snowy Edges: Using an offset spatula or the back of a spoon, gently spread the glaze evenly over the top. Then, for that "French tip" or "snowy edge" effect, gently push some of the glaze towards the perimeter of the bars, allowing it to slightly drip and gather. Don't overthink it; the natural unevenness is part of its charm.
13. Set and Cut: Let the glazed bars sit at room temperature for at least 30-60 minutes, or until the glaze has fully set. You want it firm enough that it won't smudge when you cut. Once set, use a sharp, warm knife (wipe it clean between cuts!) to slice the bars into desired shapes. I usually go for 12-16 bars.
14. Serve and Enjoy: Arrange them on a pretty platter and watch them disappear! These are truly a delightful holiday treat.