Answer: The Mishna (Challa 1:5) writes that one isn’t obligated to separate challa from a thin batter unless it takes on a thicker consistency when it is baked (See Rashi). Following this, the Shulchan Aruch (YD 329:2) writes that, providing it contains enough flour, one has to take challa when baking something from a liquid batter just as one does from a thick dough. R' Avraham Borenstein (Avnei Nezer YD 413) writes, however, that one shouldn’t separate challa until after the cake or biscuits have baked.
The Aruch Hashulchan (YD 329:15) disagrees, writing that as cakes and biscuits are not considered to be bread, there is no need to separate challa from them in chutz la’aretz when separating challa is only miderabanan.
R’ Shlomo Zalman Auerbach (Minchas Shlomo 68:n1; Shemiras Shabbos Kehilchasa 42:n41) disagrees, however. After all, if one were to eat enough cake as a meal, one would need to wash beforehand, say hamotzi beforehand and bentch afterwards. R’ Pesach Eliyahu Falk (Machazeh Eliyahu 110) writes that it is clear that the other poskim disagree with the Aruch Hashulchan, too. One must, therefore, separate challa even with a beracha.
The Shulchan Aruch (YD 329:10) writes that one should avoid baking without water or one of the other liquids that is mekabel tuma. If necessary, one should add some water to avoid this issue (See Kitzur Shulchan Aruch 35:7).
In conclusion, if one bakes a large enough batch of cake dough, one should separate challa with a beracha. One should add a little water to recipes that don’t call for water (or one of the other seven liquids). If the mixture is more of a batter than a dough, then one should wait until it is baked to separate challa.