Line square baking dish (8x8" or 9x9") with aluminum foil. Spray the foil with cooking spray.
Add butter, sweetened condensed milk, and white chocolate chips to a saucepan over medium heat. Stir occasionally until chips are melted - about 5 minutes.
Add in mini marshmallows, stir occasionally until they dissolve into the mixture - about another 2 minutes - and then remove from heat. Stir in vanilla, sugar cookie crumbs, and sprinkles.
Pour the mixture into prepared pan, ensuring you get it to spread as evenly as you can across the pan.
Immediately sprinkle extra sugar cookie crumbs and sprinkles on top.
Chill for at least 3-4 hours or overnight.
Remove pan from fridge, lift fudge out with foil, and then use a sharp knife to slice fudge into about 32 smaller pieces. You can make fewer cuts than this to produce larger pieces of fudge, but because this is such a decadent and sweet treat, I like to make smaller pieces.
Store in an airtight container at room temperature for several days. It'll stay moist & delicious for quite a while!