Variable Current Amplifier for Fan Control
You're using the BJT to drop significant voltage while passing significant current.  The power it dissipates is determined by Watt's Law: \$P=EI\$.  A better option would be to use a switching converter.  Motors are inductive and can function well with pulsed voltage, so they are often driven with PWM, where pulses of full voltage are used to simulate a lower voltage.  When the switch is only driven in hard-ON and hard-OFF modes, it does not dissipate significant power so long as it is chosen to switch quickly enough.  

For your circuit, since it has an analog input, a PWM conversion would be accomplished by using a comparator to compare the analog input(scaled with a voltage divider if necessary) to a triangle wave of suitable amplitude and frequency.  The output of the comparator is used to control a switch, so that the switch is turned on whenever the reference voltage is higher than the triangle wave.  The result in an ideal system looks like this:



The yellow bars represent the times your switch is ON, and the red is a variable analog input voltage and the blue a triangle wave.  The graph shows the switch on 60% of the time when the reference voltage is 60% and that the start and end time of the cycle will vary as reference voltage does.

So you can build your own if you're interested (google PWM fan control circuit) or attempt to buy a PWM controller or variable power supply to drive the fan from your output signal.