Preheat your oven to 350°F so it's ready when your cookies go in. Line baking sheets with parchment paper to prevent sticking and ensure even baking. Having everything ready keeps the process smooth.
In a mixing bowl, combine softened butter, sugar, and vanilla extract. Beat until light and fluffy, which takes about five minutes. Proper creaming traps air, giving cookies a tender texture.
Add the egg and mix until fully incorporated. Then gradually mix in flour until the dough forms and becomes soft but not sticky. Avoid overmixing so cookies stay tender.
Roll the dough into equal balls and place them evenly spaced on baking sheets. Press one chocolate heart gently into the center of each dough ball, creating the thumbprint shape.
Bake cookies until the bottoms turn lightly golden, about 19 minutes. Allow cookies to cool on the baking sheet so they firm up without breaking and the chocolate sets nicely.