Prepare the Oven and Baking Sheets: Preheat oven to 350°F. Line 3 baking sheets with parchment paper. This preparation ensures even baking and prevents the cookies from sticking to the sheets, making for easy removal after baking.
Mix Dry Ingredients: In a medium bowl stir together flour, baking powder, and salt. Set aside. Combining these ingredients separately ensures they're evenly distributed throughout the cookie dough, resulting in consistent texture and leavening.
Cream Butter and Sugars: In a large bowl beat butter, white sugar, and brown sugar together on high speed until light and fluffy, about 2 minutes. This step incorporates air into the mixture, which helps create a tender cookie texture.
Add Wet Ingredients: Beat in egg and vanilla until well mixed. The egg provides structure while the vanilla enhances the flavor profile of the cookies.
Combine Wet and Dry Mixtures: Pour dry ingredients into the wet ingredients and beat just until well mixed. Overmixing can develop too much gluten, resulting in tough cookies, so mix just until incorporated.
Form Cookies: Drop rounded cookies, 1½-tablespoon size and 2 inches apart, onto parchment paper lined baking sheets. The spacing allows room for the cookies to spread slightly during baking.
Create Indentations: Using the back of a teaspoon, dusted in flour to prevent sticking, press about half way down into each cookie to create an indent for the jam. This creates the signature "thumbprint" that gives these cookies their name.
Add Jam Filling: Using a small spoon, fill each indent with jam of your choice. The jam will spread slightly during baking, so avoid overfilling the indentations.
Bake the Cookies: Bake for 13 to 15 minutes or until bottom edges look slightly browned. The cookies should remain pale on top with just a hint of golden color around the edges.
Cool Completely: Remove from oven and let cool for 10 minutes before removing and placing on wire racks to cool completely. This cooling period allows the cookies to set properly before handling.
