Heat your oven to 350°F and line a baking sheet with parchment paper. This advance preparation ensures you're ready to bake as soon as your dough is chilled. While the oven heats, measure out all your ingredients so everything is ready to go—this is especially important for cookie dough where you want to work efficiently once you start mixing.

In a mixing bowl, combine the softened butter and sugar, beating them together until the mixture is light, fluffy, and pale in color—this should take about 2-3 minutes with an electric mixer. This creaming process is crucial because it incorporates air into the dough, which gives the cookies their tender texture. I like to scrape down the sides of the bowl halfway through to ensure everything is evenly mixed.

Add the egg yolk, vanilla extract, and salt to the butter-sugar mixture from Step 2, mixing on medium speed until the mixture is smooth and completely combined. The egg yolk acts as a binder and adds richness to the dough, while the vanilla and salt enhance the overall flavor. Mix just until you don't see any streaks of egg or dry ingredients—overworking at this stage won't hurt the dough, but you want to move to the next step promptly.

Add the flour to the wet ingredient mixture from Step 3 and mix on low speed until a soft dough just comes together. Stop mixing as soon as you no longer see streaks of flour—the dough should be cohesive but not overworked. I find that mixing on low speed prevents the flour from puffing up and keeps the dough tender rather than tough.

Roll the dough from Step 4 into balls approximately the size of walnuts and place them on the prepared baking sheet, spacing them about 2 inches apart. Gently press the handle of a spoon or your thumb into the center of each ball to create a shallow indentation for the jam filling. Transfer the baking sheet to the refrigerator and chill for 1 hour—this resting time is essential because it allows the gluten to relax and helps the cookies maintain their shape during baking without spreading too much.

Remove the chilled dough balls from the refrigerator and spoon approximately 1/2 teaspoon of guava jam into the center indentation of each cookie. Place the baking sheet in your preheated 350°F oven and bake for 10-12 minutes, until the edges and bottoms are light golden brown while the centers remain slightly soft. The jam will soften and settle into the thumbprint as it bakes, creating a beautiful presentation.

Allow the baked cookies to cool on the baking sheet for 10 minutes—this helps them set and become sturdy enough to move without breaking. The residual heat from the baking sheet will continue to gently cook the bottoms, ensuring they're fully set. After 10 minutes, you can transfer them to a wire rack or serving plate if desired, though they're delicious warm or at room temperature.