Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter with peanut butter and sugar until fluffy. Add in egg and vanilla extract until smooth.
Add in the flour mixture, mix until it is fully combined. Roll the cookie dough into balls, and place them onto a large ungreased baking sheet at least 2-inches apart.
Place conversation hearts on each cookie. Bake for 10-12 minutes.