Halacha: What Blessings Are Recited Over Cornflakes?

On the cornflakes, we say 'Shehakol' (first blessing)

And at the end of the meal, we say 'Borei Nefashot' (concluding blessing)

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The serving size required to be obligated in a concluding blessing (from "Daily Halacha" by Maran Rabbi Ovadia Yosef, of blessed memory):

For anything that a person eats, one must recite a "first blessing" such as the blessing "Shehakol" or "Ha'etz." Even if one eats only a small amount of that food, it is required to bless, as long as the purpose is to eat it. (However, if the intention is merely to taste it in order to adjust seasoning, for example, no blessing is made for such tasting). Therefore, if someone chews flavored chewing gum, even if they swallow only a small amount, they must nonetheless bless for deriving pleasure from the gum's taste. Similarly, anyone who drinks even a single drop of a beverage must bless it.

However, all this applies only to the first blessing, whereas a concluding blessing is only spoken when one has consumed an "important serving." An important serving is defined as the quantity called a "kezayit," which is 27 grams, and in the context of drink, the measure is "revi'it," which is 81 grams.

Additionally, eating or drinking should occur within a time frame where consumption is linked. If, for example, one consumes an olive-sized amount of bread, piece by piece, throughout an entire day, it is clear one would not subsequently recite Birkat Hamazon. The entire olive-sized serving needs to be consumed within the time frame known as "eating a small portion." There is a debate among rabbis regarding the length of this time frame—some say it is five minutes, while others suggest approximately seven and a half minutes (there are other opinions as well). Thus, it's always advisable to consume a kezayit (especially during the Shabbat meals) within four and a half minutes to satisfy all opinions.

Regarding the concluding blessing for drinks, one should drink a revi’it of the liquid (81 grams) in one go. If one drinks with pauses between gulps, no concluding blessing is recited. Hence, someone drinking soup with a spoon doesn’t recite a concluding blessing for it, as they don’t consume a revi’it at once. The same goes for drinking hot coffee or tea; no concluding blessing is recited because it's not possible to drink it all at once.