Write CBCharacteristic by UUID

Write CBCharacteristic by UUID

I am attempting to use CoreBluetooth to write to a specific, known
characteristic. I feel this should be possible as I have used a Texas
Instruments BLE utility that you can select a "write value" action on a
connected peripheral and simply enter the characteristic UUID and the
value you want written and it executes no problem.
By my understanding, in order to do this I have to make a call to
[peripheral writeValue:data forCharacteristic:...];
with a CBCharacteristic object configured to have the correct UUID.
I have tried making a CBMutableCharacteristic with the correct UUID, and
even correct permissions for what I know is an existing characteristic in
this peripheral's profile, and I get the following crash, when attempting
to perform any read/write operation with that characteristic on the
peripheral:
*** Terminating app due to uncaught exception
'NSInvalidArgumentException', reason: '-[CBMutableCharacteristic
peripheral]: unrecognized selector sent to instance 0x1ed57690'
And here is the code I use to set up and write to the characteristic:
NSString *characteristicUUIDstring = @"fff1";
CBUUID *characteristicUUID = [CBUUID
UUIDWithString:characteristicUUIDstring];
char dataByte = 0x10;
NSData *data = [NSData dataWithBytes:&dataByte length:1];
CBMutableCharacteristic *testCharacteristic = [[CBMutableCharacteristic
alloc] initWithType:characteristicUUID
properties:CBCharacteristicPropertyRead|CBCharacteristicPropertyWrite
value:data
permissions:CBAttributePermissionsReadable|CBAttributePermissionsWriteable];
[peripheral writeValue:data forCharacteristic:testCharacteristic
type:CBCharacteristicWriteWithResponse];