Preheat oven to 375 degrees Fahrenheit. Line an 8x8 baking dish with parchment paper or lightly spray it with oil.
In a large mixing bowl, add all ingredients (except diced apples and Topping) and stir well, but do not over-mix.
Transfer the cake batter into the baking dish, gently spreading it out to be level.
Top evenly with the small-diced apple pieces.
In a small bowl, add all the topping ingredients (do not melt the vegan butter). With a fork (or hands), mash the ingredients together until it becomes crumbly with some chunks, where the everything is incorporated.
Spread the topping mixture evenly over the cake.
Place the baking dish into the oven, and bake for 38 to 40 minutes. A toothpick should come out clean when it's ready.
Let cool for 10 minutes before removing from the baking dish and transferring to a cooling rack.
Let cool a further 20 minutes before serving.