The process sounds like a foam-injection station (which I used to operate in a plastic manufacturing plant). In this case, there are two cylinders of material.
You haven't indicated the pressure you need for your valve; this is important. You should be able to find these valves for different industries depending upon the material (hydroponics, beverage-dispensing systems).
I'm not sure how you would package the chocolate, for example, so that it may be extruded. Nobody sells chocolate filament.
3D printed chocolate
I imagine that it would be difficult to keep a batch of chocolate very long after it has been made ready to print.
The problem with an OctoPrint-based solution would be that this is a firmware-based problem. OctoPrint sends commands over to the printer controller and it extrudes when it wants to extrude. OctoPrint doesn't know when the printer is running which line of gcode. So if you try to use an OctoPrint plugin to control a valve the timing will be off.
Assuming for a moment that you have a custom firmware what you could do is to adjust the gcode to add another tool.
G1 X0 Y100 E20
This could be adjusted to:
G1 X0 Y100 E20 A10
In theory, the firmware could interpret "A10" to mean that it should inject (extrude) a certain amount of the nitrogen or whatever gas that you're trying to use. The timing now is correct; the printer will do both things at once.