Preheat the oven to 180C/350F and grease and line an 8x8 inch square pan. I use Wilton Bake Easy to spray the pan, then put two strips of grease proof paper each way across the pan, so I have something to lift the cake out with.
Make the crumb topping: Place butter, sugar, oats and flour into a medium sized bowl and rub together until you have a coarse breadcrumb texture; some larger lumps are fine, these get super crunchy! Leave to one side.
Place chunks of apple, flour, salt, spices and baking powder into a medium sized bowl and stir until all the apple chunks are coated in flour.
Place butter and sugars into a large bowl, or the bowl of a stand mixer and beat until light and fluffy, around 2 minutes on medium-high speed.
Add in the eggs and vanilla, scraping down the sides as necessary, and mix on medium speed until well combined and smooth.
Add in the flour mixture and pumpkin and mix on low until the mixture comes together and there are no lumps of flour.
Tip the batter into your prepared pan and smooth into an even level with a spatula/wooden spoon.
Sprinkle the crumb topping all over the top of the cake and then place in the oven for 25 minutes. After 25 minutes, turn the oven down to 160C/325F and bake a further 20-25 minutes until risen, golden and an inserted skewer into the centre of the cake comes out clean.
Leave cake to cool completely, before transferring to a cutting board to slice into chunks of heaven. You can also make a quick glaze of 1/2 cup (60g) powdered sugar and 1-2tbsp water. Drizzle over the top if desired.
Pumpkin Apple Coffee Cake will keep in an airtight container, at room temperature for 3 days. The crumb top will get softer after 2 days.