Is this a USB dongle or internal integrated module for a PC, Laptop or similar application?
The problem is you would need to provide a HCI USB Stack on the MCU, which is definitely a nontrivial endeavor.
What family of MCUs were you considering?
If you had an embedded processor board capable of running Embedded Linux, Embedded Windows or Windows CE, this would most likely be the least painful route.
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Here's a link discussing the implementation of HCI over USB via PIC24 or PIC32, if you are set on using a MCU you will have to use a higher end device:
How to control a Bluetooth USB dongle by PIC24FJ64G002 on HCI protocol layer