I want to describe a specific type of cake which is very popular in Sweden (kladdkaka). It contains no baking powder, and therefore the consistency remains almost viscous, kind of like fudge. In English I think "gooey" suits. What can I say in Esperanto?
I think the word glueca is what you're looking for. The PIV defines it as follows:
glueca. Prezentanta la ecojn de gluo: rezino estas glueca; gluecaj manoj de infano.
However, if you want to emphasize viscosity (which might not make sense for a cake that, while sticky, is still solid), you could use viscoza or viskeca:
viskoz/a ☐ Malfacile fluanta, siropeca alteniĝema, kiel la fruktosuko de visko. Sin. viskeca.