This is actually a machlokes based on what is your preference as well as what is considered the ikar and tafel in the product as well as the majority.
This seems to be analogous to the case of Rav Moshe Feinstein discussing what bracha to make on chocolate covered raisins.
A Sweet Bracha What Bracha Does One Recite over Chocolate-Covered Raisins?
In Rav Moshe’s tshuvah, he addresses the following issue: When eating
a food composed of items with different brachos, we must determine
which food is the more important part, the ikar, and determines the
bracha of the entire food. Rav Moshe deliberates whether the chocolate
or the raisin is more important in order to determine whether the
bracha on chocolate-covered raisins is Ha’eitz, like the raisin, or
Shehakol, like the chocolate. Rav Moshe concludes that neither the
chocolate nor the raisins can be considered of secondary importance
(tafeil) to the other, and therefore chocolate-covered raisins require
two brachos, Ha’eitz on the raisins and Shehakol on the chocolate.
Rav Moshe then discusses which of the two brachos to recite first.
Usually, one should recite the bracha of Ha’eitz before reciting
Shehakol. However, Rav Moshe points out that one must eat the
chocolate before reaching the raisin; thus, the bracha on the
chocolate will have to be first. Rav Moshe concludes that the best
thing to do is to recite Ha’eitz on a regular raisin and then Shehakol
on the chocolate. (When this option does not exist, he paskins that
one should recite Shehakol on the chocolate and then Ha’eitz on the
The way that I read this is to make the shehakol immediately followed by haeitz (in the case of a raisin) or mezonos (in the case of the Twix or Kit Kat bar) and then eat the two simultaneously.
There are other ways of handling the matter.
For example, OU Guide to Blessings cited at [Brachot 38 – Which blessing on a twix?] says that one can make the shehacol first deliberately excluding the wafer part and then making mezonos to include the wafer. After this eat the two together without a hefsek. This is similar to Rav Moshe's psak, but he does not appear to require that one explicitly excludes the wafer (or the raisin) from the shehakol.
If you eat a candy bar containing fruit, nuts, wafers, or other
fillings because you specifically want to eat the filling, you should
say only the appropriate brachah for the filling. If, however, the
chocolate itself is just as important to you, first say Shehakol for
the chocolate (bearing in mind that you do not intend to include the
filling in this brachah), and then the appropriate brachah for the
filling (Igrot Moshe, Orach Chaim 3:31).
This does not apply in the
case of wafers, since the chocolate is always considered a subordinate
ingredient in a candy bar with wafers, and therefore, only Mezonot
need be said (Mishnah Berurah, Orach Chaim 168:27).
Bracha on chocolate with wafer says that it is really indeterminate.
Question: If a chocolate seems to consist mainly of wafer (eg kif-kef or Kit-Kat) should one say "Mezonot?" I assume that if there
is doubt concerning whether there is more chocolate or wafer one
should say "Shehakol"?
Answer: Though there is not universal agreement on this point, it seems the most common positions that if the majority is chocolate, it
is "Shehakol" and if the majority is wafer it is "Mezonot". Some hold
that since we eat the wafer for the chocolate and the wafer is just to
hold it together, it is always "Shehakol" while others argue that the
wafer also tastes good (sometimes) and therefore the rule that
"Mezonot" is dominant even when it is not the majority should apply.
It appears that the advice given in the first location cited is to say
As far as I can tell – there is no consensus. You could take the easy
route and not eat such chocolate bars/cakes ever again chas veshalom
or weigh up the above positions and decide for yourself. Maybe even
ask your Rabbi what he does!