Preheat the Oven: Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
Mix Wet Ingredients: In a large mixing bowl, combine 1 cup peanut butter, 1 cup granulated sugar, 1 egg, and 1 teaspoon vanilla extract. Beat until smooth and creamy.
Add Dry Ingredients: Sift in 1 cup all-purpose flour and 1 teaspoon baking soda. Stir the mixture until the dough is fully combined.
Shape the Dough: Roll the dough into 1-inch balls and place them on the prepared baking sheet, spacing them about 2 inches apart. Flatten each dough ball with a fork to create a crisscross pattern.
Bake the Cookies: Bake for 8-10 minutes, or until the edges are golden brown. Remove from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
Add the Jelly: Once cooled, create a small indentation in the center of each cookie and fill with 1 teaspoon of your favorite jelly.