In a medium saucepan, whisk together all pudding ingredients, except for the vanilla. Turn the heat on medium and cook, stirring frequently, until thickened – about 7-9 minutes. After about 5 minutes, start to stir constantly. It will go from thin and watery to thick in a matter of seconds. Do not let the pudding boil! It's okay if it boils for a few seconds, but not more. Stir in the vanilla. It will be quite a bit thicker than regular pudding and will thicken even more as it cools. Set aside to cool while you prepare the cake. If you want to prevent a film from forming, cover with a piece of plastic wrap. I skip that step and just give it a good stir after it's cooled. There will still be some lumps, but you don't notice them in the finished cake.

In a small mixing bowl, stir together the flour, sugar, cinnamon and salt. Incorporate the coconut oil until well combined. If the mixture isn't firm, place in the refrigerator to chill while preparing the rest of the cake.

Preheat the oven to 350 °F (175 °C) and place a piece of parchment paper in a 9" springform pan and press it down to conform to the pan's shape.

Mix the apple slices, sugar and cinnamon together. Set aside.

In a medium mixing bowl, stir together the flour, baking powder, baking soda and salt. Set aside.

In a large mixing bowl, beat the coconut oil and sugar at medium speed until well combined - about 1 minute. It won't become light and fluffy like when you cream butter and sugar together. This will be more crumbly. Beat in the vanilla and egg yolk on low and mix just until combined. Add half of the flour mixture and beat on low until mostly combined. Stir in the buttermilk until combined and then the remaining flour mixture. Stir just until combined. Do not over mix!

Spread the batter onto the bottom of the prepared springform pan. Spoon the pudding as evenly as possible over the cake batter. Use a spoon to very gently spread it, if necessary. Arrange the apples over the pudding and then sprinkle the streusel evenly over the top.

Bake for 30 minutes. The center should only jiggle a little when tapped. The toothpick test doesn't work here due to the pudding.

Let cool completely and serve. Cover and refrigerate any leftovers for up to 3 days. It's okay cold but tastes better once it's warmed to room temperature.