Mix the butter and sugar.
Stir the softened butter and sugar together until smooth and creamy. It should look pale and spread easily.
Add the flour.
Mix in the flour until a soft dough forms. The dough should hold together and not feel sticky.
Shape the dough.
Roll small pieces of dough into balls and place them on the baking sheet. Leave a little space between each one.
Make the thumbprints.
Press your thumb gently into the center of each dough ball. You want a shallow dip, not a hole through the cookie.
Fill with jam.
Spoon a small amount of jam into each dip. The jam should sit inside without spilling over.
Bake the cookies.
Bake until the bottoms look lightly golden and the tops stay pale. The cookies should feel set but still soft.
Cool and glaze (optional).
Let the cookies cool. If using glaze, drizzle it lightly over the tops once they are cool.
