Why does my cake have to be inverted? Why a 2-layer appearance?

I am a late starter in baking. I am trying to make yam cake, which we call "ube cake" here.

Some recipes suggest that after baking, immediately invert the pan over a wire rack. Why is it so? Doing so results in vertical parallel lines appearing across the face of the cake. How do I remedy that?

The recipe also calls for cake flour. I use a substitute of all-purpose flour + cornstarch. What will be the result if I will just use all-purpose flour?

I did two trials:

1st trial: I mixed the dry ingredients (substitute cake flour, baking soda, salt, sugar) and then poured the wet ingredients (unmixed - yam, oil, milk, egg yolk, color) over the flour mixture. Then I combined dry & wet ingredients but they were rather tough to mix, because it became thickened. The egg white was mixed separately with cream of tartar & a tbsp of sugar, and folded in. The baked result turned out okay though the surface was strangely brownish.

2nd trial: The sugar in the dry mixture was decreased from 4 tbsp to 3 tbsp). I increased the yam portion a little, (from 3-1/2 tbsp to 4 tbsp). Then I mixed (using egg beater) the yam along with the other wet ingredients (oil, milk, egg yolk, color). After that, I poured the flour mixture into the wet mixture in 2 batches, using an egg beater to mix the combined ingredients. I did not experience the dough-like consistency as I did in the first trial. The sugar in the egg white mixture was increased from 1 tbsp to 3 tbsp.

The second trial resulted in a two tiered appearance. The upper portion is fluffy, while the lower portion was dense. Why was it like that?

• Was it due to mixing all the wet ingredients to included the mashed yam?
• Was it due to increased portion of the yam from 3-1/2 tbsp to 4 tbsp?
• Was it due to over mixture of the egg white?
• Was it due to increase of the sugar in the egg white mixture?
• Was it due to putting dry ingredients over wet ingredients?
• Was it due to using 2 batches of the dry ingredients to pour into the wet ingredients?
• Was it due to overall over mixing?
• Should the yam not undergo beating anymore?

Also, how do I prevent the surface from being brown? The inside was purple though. I hope you will bear with my numerous questions.