Whisk the whole milk and sweetened condensed milk together in a large measuring cup until smooth.
Add the heavy cream, pudding mix, malted milk powder, vanilla, and salt. Whisk vigorously for 30 seconds, until the mixture thickens slightly.
Pour into two ice cube trays, leaving a little room at the top. Freeze at least 4 hours or overnight, until fully solid.
Add 10 to 12 frozen Frosty cubes and the cold brew concentrate to a blender.
Blend on high for 20 to 30 seconds, scraping down the sides once, until smooth and pourable.
Adjust as needed: more cold brew to thin, more cubes to thicken.
Pour into two tall chilled glasses, top with whipped cream and drizzle if using, and serve immediately.