Level 840 is the tenth level in Nougat Noir and the 365th jelly level. To pass this level, you must clear 16 single and 16 double jelly squares in 30 moves or fewer.
When you complete the level, Sugar Crush is activated and will score you additional points.
The board layout forces the creation of vertical
striped candies or special candy combinations to destroy the waffles at the beginning of the level.
The presence of four candy colours makes this a little easier. The jellies are worth 48,000 points, which is more than the one-star target score. 
Even though you have 30 moves available, you should still use it very wisely.
It is recommended to make vertical striped candies to destroy lots of waffles in the level. Making a colour bomb + striped candy combination is way much better.
Then make as many special candies as possible to remove all the jellies and the waffles.
This level is shaped like an hourglass. The other is level 322, which is also shaped like an hourglass.
↑ (16 double jellies × 2,000 points per double jelly) + (16 single jellies x 1,000 points per single jelly) = 48,000 points
Candy Crush Saga Level 840 (Jelly level) - 3 Stars Walkthrough, No Boosters