Select the icing, find the Particle panel.
- Click + to get a new particle set, and change it to Hair.
- Under Render, change Render As from Path to Object and pick the object. Boom.
- Scale up/down to get the size you kind of want.
- Scale Randomness isn't great for sprinkles since you want uniform thicknesses.
- Under the "Hair" indictor hit Advanced so we can do something about the direction.
- Rotation, check the box, change Orientation Axis to Normal.
- Not (much) Randomize 'cause that'll make them stand up, but Randomize Phase instead.
- Emission -> Number, obviously that's how many dingdang sprinkles we want.
- Sprinkles showing up underneath the icing? Weird! Time for Weight Painting!
When you've done Weight Painting make sure to select Density = Group to apply what you've done.