Preheat the oven to 350°. Line a baking sheet with a silicone baking mat or parchment paper.

Mix the peanut butter and the sugars together in a large bowl. Beat with an electric mixer for 2-3 minutes. Add the egg and vanilla and beat for a few more minutes. Add the salt and baking soda and stir just until combined.

Use a cookie scoop to scoop out tablespoonfuls of the dough and shape them into balls. Place on the baking sheet and press an indentation in the middle of each dough ball. You can use your thumb or the back of a tablespoon.

Bake the cookies for 10-12 minutes. I tend to bake cookies on the shorter end of the range because I like them a little bit underdone, but if you like them a little bit on the crispier side, you can leave them in a minute or two longer.

As soon as you take the cookies out of the oven, use a tablespoon to press down the indentation a little bit more and then let the cookies cool for 2-3 minutes on the baking sheet. Transfer the cookies to a wire rack and cool completely.

When the cookies are cool, add a spoonful of marshmallow cream to the top of each cookie. The easiest way to do this is to use two spoons. Scoop with one of them and then push it off the spoon with the other spoon.