Now from here we could use Proportional Editing to add our dribbles to our Icing, but you might notice that we get a bit of overhang when we attempt to do so. Which is why we need to have our Icing snap to our Doughtnut's surface.

Click on the Snapping Button at the top of the 3D Viewport, and under the section Snap Target for Individual Elements choose Face Project.

Once Face Project is selected, click the Snap Button next to Snapping, and the use Praportiaonal Editing to add dribbles to our icing.

We can see now that when we go to use Praportional Editing to begin adding dribbles to our icing, that our Icing snaps to the surface of our doughnut.