Piping cream through a piping tip

Flip Fluid addon [$]

• create star (you can use Extra Objects addon) and animate movement
• under Physics Properties enable Flip Fluid > type Inflow
• add Cube object and scale it to Domain dimension
• under Physics Properties enable Flip Fluid type Domain
• under FLIP Fluid World enable Viscosity
• Bake

Credits for Cup-Cake model goes to MrSorbias.

Mantaflow

in general same system (domain / inflow). I just wasnt able to setup enough viscosity without strange behaviour of particles. This area doesn't seems to be covered in Mantaflow solver. You can find a parameters in Physics Properties editor under Diffusion, but from what I saw on net, most thick liquid is still too far from cream with ability to keep a form.

For more details check this intro

Being a curve addict here is a solution using bezier objects for this.

Create three sets of bezier curves:

1. Section shapes for varying cylinder tips
2. Piped cream objects themselves
3. Taper shape(s)

Create varying bezier curve shapes for the foreseeable piping bag tips you'll be using. These were created from mesh circles with different number of segments and converted to bezier curves after the fact.

Create a taper object. This curve will be used to control the shape/width of the piped cream along its length. Position along the Y axis of the mid vertex will set the width of the puff at its widest point.

Create a bezier curve object for the piped cream itself. Optionally give it a twist with the Tilt function (Ctrl + T in Edit Mode). Adjust its shape length and twist as desired.

Set the Taper Object to the taper curve created above and set the Bevel Object to one of the desired sections.

Turn on the option Map Taper so the bevel isn't clipped but instead "slides" along its length.

Now animate the End Bevel property to simulate the squirting action.

For added dynamism you can create and animate the influence of additional shape-keys in both the taper and main curves, to simulate gravity squashing and the elasticity of the cream.

To expand on my comment, and if you only need the moment the cream passes through the tip, an easy way to fake it would be to pass a cylinder through a lattice.

Start with a cylinder with a few edge loops and a subdivision modifier. Add a Lattice with an uneven number of U and V divisions and 3 W division. Scale the middle W to zero, then make the bottom of the lattice into a star shape matching the shape of you tip.
Add the lattice modifier to the cylinder and now, moving it along the Z axis will deform it as if it'd been through the tip.

Now, this is obviously just a quick and simple way to fake it, and you might need to use more complicated softbody or fluids simulations if you want more realism. But I thought it was worth pointing out this technique.