Preheat oven to 350°F/177°C. Line a baking sheet with parchment paper.
In a large glass bowl, add the melted salted butter, SunButter, egg, sugar, and vanilla extract. Whisk until everything is combined.
Add the flour and baking soda to the butter mixture and use a spatula to mix until it forms a dough. (If you press the dough together, it should stick nicely into a ball and not crumble apart. If it appears too "wet," add 1 tablespoon of flour and mix again.)
Add in the marshmallows and crushed pretzels to the cookie dough and mix to combine.
Use a small ice cream scoop to portion the cookie dough into 12 balls. Place onto a parchment-lined baking sheet.
Press in additional marshmallow and pretzel on the tops of the cookies and chill in the refrigerator for 10-15 minutes.
Bake the cookies for 8-10 minutes at 350°F/177°C, until slightly puffy, dry, and golden in colour. Don't overbake.
Leave the cookies to cool on the sheet for 5 minutes before transferring to a cooling rack.
Enjoy warm, or cool completely and store in an airtight container for up to 1 week.