CHILL THE FROSTED CAKE before you add the chocolate drips. The cold buttercream will help slow the drips for more control.
Make sure the chocolate drizzle is at the CORRECT TEMPERATURE. It should be fluid, but not warm or hot. Let it cool to room temperature or chill it in the refrigerator.
TEST THE DRIZZLE by making a practice drip or two on the back of the cake before adding it to the entire cake. The glaze should flow slowly and smoothly. If it slips right off the cake, it is too warm. If it clumps and does not drip, it is too cold.
CONTROL THE DRIPS. Instead of pouring the drizzle on top of the cake, add drips individually around the sides of the cake. You can use a small spoon for this or a squeeze bottle.
Once the sides are dripped, pour the remaining drizzle on top of the cake and GENTLY SMOOTH IT OUT with an offset spatula until it meets the edges. Again, make sure the drizzle is at the correct temperature. At this point, it may have cooled too much and you may have trouble smoothing it evenly. Gently reheat as needed.