Preheat your oven to 350°F (180°C, or 160°C on a fan-forced setting). Line a large baking sheet with parchment paper—I use this instead of greasing because it prevents the bottoms from browning too quickly. This simple step takes 2 minutes and saves cleanup.

In a medium mixing bowl, combine your softened butter and powdered sugar. Using an electric handheld mixer on medium speed, beat them together for about 2 to 3 minutes until the mixture is pale, fluffy, and noticeably lighter in color than when you started. If you're using a stand mixer with the paddle attachment, this takes about the same time. You'll notice the mixture goes from a compact ball to something almost mousse-like—that's exactly what you want. The air you're incorporating into the butter is what creates that tender crumb.

Pour in your vanilla extract and mix on medium speed for about 30 seconds until it's completely incorporated and the mixture looks smooth and even. Don't skip this step—vanilla adds a subtle depth that keeps people from guessing exactly what makes these biscuits so good.

In a separate bowl, whisk together the all-purpose flour, cornstarch, and salt. Whisking them together ensures the cornstarch is evenly distributed (this prevents clumpy texture) and the salt is mixed throughout so you don't get salty pockets. This takes just 1 minute, but it matters for consistent results.

Here's where I want you to slow down and be gentle. Add the dry ingredients to your butter mixture in three separate additions, mixing on low speed (or with a wooden spoon if mixing by hand) for just 15 to 20 seconds after each addition. Stop mixing the moment you don't see streaks of flour anymore. Overworking the dough makes tough biscuits instead of tender ones. After the third addition, the dough should come together in a soft, cohesive ball.

Using a cookie scoop (about 1 tablespoon size), scoop balls of dough directly onto your prepared baking sheet, spacing them about 2 inches apart. If you like a sparkly finish (and honestly, who doesn't?), gently roll each ball in granulated sugar before placing it on the sheet. This step takes about 5 minutes for the whole batch and adds such a pretty touch.

Using your thumb (and only your thumb—it creates the perfect-sized indent), slowly press into the center of each dough ball, pushing down about halfway through to create a well. Be gentle and deliberate here; you're not stabbing, you're gently nestling your thumb into the dough. The goal is a deep pocket that won't disappear during baking but that still leaves borders so the curd doesn't leak out.

Spoon about ½ teaspoon of lemon curd into each well. If your lemon curd is very thick, you can thin it slightly with a few drops of water to make it easier to spoon. Don't overfill—the curd will stay put better with a modest amount, and the flavor will still shine.

Slide your baking sheet into the preheated oven and bake for 10 minutes, or until the edges are just barely starting to turn golden brown. The centers will still look quite pale—this is correct. Overbaking will make them hard instead of tender. At exactly 10 minutes, pull the sheet from the oven. They'll still feel quite soft, which is perfect.

Leave the biscuits on the baking sheet for 10 minutes to set slightly and firm up. This resting period is crucial—it lets the structure develop so they won't fall apart when you move them. After 10 minutes, transfer them to a wire cooling rack to cool completely, about 15 to 20 minutes. Once cooled, the lemon curd will have set and the biscuits will have that perfect tender-but-not-fragile texture.