I bake them in a 350 degree oven for 45 minutes but it seems the insides refuses to harden and dry any advice?
PS: The mixture is (oat flour, baking powder, mashed banana, Jam, milk, maple syrup, honey, vanilla extract, nuts)
not sure what's causing your problem, so need some more info. On flour, are you using ONLY oat flour? That itself may be a problem, as oat alone have or make when baked the long strings of gluten that help wheat flour products hold together. That's why most specialty flours, no matter how tasty or otherwise nutritious, are mixed with wheat flour for baking. Second area to look at is whether proportion of liquid to dry ingredients is too high. Could be a typo or an error in measuring. Perhaps re-read and/or re-make a (small) batch?
Should read "oat alone does not have or make the long strings of gluten"
I agree with Nancy. I have to bake gluten-free. And when I look at the ingredients in your recipe, my first thought is that you have no binder, which would be the gluten found in wheat flour. There are a lot of binder substitutes - eggs, xanthan gum, starches like tapioca, potato, etc.
My second thought was also the same - that's a lot of liquid. Although non wheat flours often need more liquid to hydrate the flours, since you don't have much to bind things together it may become a problem.
Did you use a recipe? It would be interesting to take a look at it if you post it.