Preheat oven to 350°F (177°C) and line two baking sheets with parchment paper.
In a large bowl, cream together the butter and sugar until light and fluffy, about 2 minutes.
1 cup unsalted butter, 1 cup granulated sugar
Beat in the egg and vanilla extract until smooth and glossy.
1 large egg, 1 teaspoon vanilla extract
In a separate bowl, whisk together the flour, baking soda, and salt.
2 cups all-purpose flour, 0.25 teaspoon baking soda, 0.5 teaspoon kosher salt
Gradually add the dry ingredients to the butter mixture, stirring until just combined.
Fold in the miniature marshmallows, stirring gently to avoid breaking them.
3.5 cups miniature marshmallows
Scoop the dough into 1½-inch balls and arrange 2 inches apart on the prepared baking sheets.
Bake for 10–12 minutes, until the edges are slightly golden.
Let the cookies cool on the baking sheet for 5 minutes.