Combine butter with both types of sugar in the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed for about 2 minutes until slightly fluffy and lightened in colour by a shade. Stop to scrape down the sides of the bowl a few times during mixing.
Add egg and vanilla and mix until well combined. Mix in peanut butter. Mix in baking soda and salt, then add the flour and mix on low until evenly combined. Cover the bowl and refrigerate for 1 hour.
Preheat oven to 350°F. Line 2 large baking sheets with parchment paper.
Scoop up heaped tablespoons of dough and roll into smooth balls. Roll them in sugar to coat evenly. Place onto prepared baking sheets spacing them 2 inches apart. Bake for 12-15 minutes until puffed and the tops are cracked and lightly golden.
When they are finished baking, immediately press a solid milk chocolate heart right into them middle of each cookie while they are still hot. Transfer cookies to a rack to finish cooling. Then, once cool, drizzle with dark chocolate and decorate with sprinkles!