In a standing mixer with a paddle attachment, cream the butter and both sugars until fluffy, about 1 minute. Beat in the egg, vanilla extract, and salt, and mix to combine.
Remove from mixer and fold in flour, baking soda, and old fashioned oats. Once combined, fold in raisins.
Scoop dough into 1-inch balls, rolling between your hands to make them perfectly circle. Place on prepared baking sheets, leaving 1.5 inches between each cookie.
Bake for 11-12 minutes. Cookies will look underdone and still pale, but this is what you want! Allow to cool on baking sheets for 10 minutes before transferring to wire racks to cool completely.
To make the frosting, combine cream cheese and butter in a standing mixer and cream until light and fluffy, about 2 minutes. Add in powdered sugar and vanilla extract and whip for an additional 30 seconds. If it’s too dry, add in 1 tablespoon of milk.
Assemble the cookies by piping 2-3 tablespoons of frosting on the flat side of one cookie, then sandwich with another cookie! Repeat with remaining cookies.