Why does one object cut through and show in another object? I am new to blender and I am trying to extrude and bring down the dribbling parts of frosting on a donut, but when I bring the vertices down, they cut into the donut as shown (picture is taken from inside the donut). 

I have proportional editing on, the snap to face tool (with project individual elements), and I even tried using ctrl button when dragging down vertices (I saw someone say that can help it to not show through - I'm not sure the reasoning of why it can work - it worked well with grabbing vertices but not extruding them for me) However, the frosting still pierces the donut - Is this normal and I shouldn't worry about it, or am I doing something incorrectly?

I appreciate any answers - Thank you!]1
I don't see a huge problem, but I can understand that you want to get rid of it. When you have proportional editing enabled,, snapping only has an effect on the selected vertices, not on the ones being dragged along. Also snapping only works if you use g or the widget to move your vertices. If you want your freshly extruded faces to snap to your topology, release them from extrude move mode and use the tools to move them.