Preheat your oven to 350°F and line baking sheets with parchment paper. Sift the powdered sugar for the filling to remove any lumps—this ensures a smooth, professional-looking glaze. Bring your egg yolk and butter to room temperature if they aren't already, as this helps create a smoother dough with better texture. Have all your ingredients measured and ready before you begin mixing.

In a mixer, beat the softened butter and sugar together until the mixture is light, fluffy, and pale—this takes about 2-3 minutes and incorporates air for a tender cookie. Add the room-temperature egg yolk and vanilla extract, mixing until fully combined. Gradually add the flour and salt mixture in two additions, mixing on low speed just until the dough comes together. I prefer to stop mixing as soon as the flour disappears to avoid overworking the dough, which can make cookies tough.

Roll the dough into 1-inch balls and place them about 1 inch apart on your prepared baking sheets. Using your thumb or the handle of a wooden spoon, press a deep indent into the center of each ball—press firmly enough to create a well for the filling but not so hard that you crack through to the bottom. The impression should be about the size of a dime and deep enough to hold the glaze without it running off.

Bake the cookies for 10 minutes until they're lightly set but still soft. Remove them from the oven and immediately press into the centers again to deepen and reshape the thumbprints—the dough will have slightly puffed during baking. Return to the oven for 8-10 minutes more until the cookies are just barely golden on the edges. The centers should still look slightly underbaked, as they'll continue cooking slightly as they cool.

Let the cookies cool on the baking sheet for 2-3 minutes until they're firm enough to move but still warm, then transfer them to a wire cooling rack. This short resting period allows them to set slightly while the centers remain slightly soft, making them easier to handle without breaking. Let them cool completely—about 10-15 minutes—before filling, as this prevents the glaze from melting.

While the cookies cool, whisk together the powdered sugar, corn syrup, vanilla extract, and milk in a small bowl until smooth. Start with 2-3 drops of green food coloring and stir well, adding more drops gradually until you reach your desired Grinch green shade—I like a slightly deeper green for better visual impact. Using a small spoon or piping bag, fill each cooled cookie's thumbprint generously with the green glaze, then top immediately with a red heart sprinkle for that iconic Grinch look. The glaze will set as it cools, creating a beautiful festive treat.