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)
This recipe is way too low in gluten, or even gluten free. You need wheat flour (or flour of other very closely related species such as spelt) to get a batter which can rise and bake normally. Any flour which does not contain gluten is not capable of trapping the bubbles created by the baking powder, and the moisture of the batter cannot escape the dough through the solid, bubbleless batter. The mashed banana and jam made it worse.
The best thing to do would be to add pure gluten in proportions common to AP flour (9 g gluten to 91 g oat flour). You will get normal batter.
If you don't want to do that, you can try to engineer a gluten free recipe using xanthan gum or other thickeners capable of producing a gloopy texture. The engineering process will take anywhere up to a dozen tests to get the texture right, and you will need to work with precision (normal scale to weigh all ingredients, small-amounts scale for the xanthan intself). Most home cooks prefer to use an existing recipe instead of going through this.
With gluten free flour blends, it's best to reduce cooking liquids by about a third to get baked products of the correct texture, but each flour blend is a little bit different in terms of how much water it needs. Also, they need to be allowed to cool all the way down, as gluten free flours tend to stay very moist when fresh baked, but lose moisture very quickly when kept (Next day muffins are always dry). They could be still cooling when you try and get them out of the pan, and since the flour (sans gluten to hold it together) is fragile, will tear. Try leaving them for half an hour to cool and see the difference in their internal texture. I would definitely use sugar instead of liquid sweeteners. It helps the flour to get crisp and gives a better texture. Finally, there are lots of allergy free recipes out there that use oats and are egg free-using either banana or lindseeds as binders. You can have a look at their ratios of flour to liquid.