For the filling:
In a medium saucepan, whisk together all the filling ingredients, except the vanilla.
Heat it on low-medium and cook, stirring, until thick, for about 7 minutes. Do not boil. Add vanilla, let cool, cover and set aside.
For the streusel:
In a medium bowl combine the flour, sugar, cinnamon and salt. Stir in coconut oil until well combined. Refrigerate.
For the apples:
Combine the cubed apples, sugar, cinnamon, lemon zest and lemon juice in a bowl. Mix and set aside.
For the cake batter:
Heat oven to 200 C.
Mix the buttermilk ingredients and set aside.
In a large bowl add flour, baking powder, baking soda and salt. Set aside.
In another bowl mix coconut oil and sugar with a mixer for about a minute. Add the vanilla and egg (or psyllium husks for vegan version) and mix for a few more seconds. Carefully add the flour mixture and “buttermilk” and mix on low power until well combined, for about a minute.
Cover your baking pan with baking paper.
Spread the batter onto the bottom of the prepared pan.
Spoon the filling evenly over the batter with a spoon.
Arrange the apples over the filling cream and then sprinkle the streusel evenly over the top, breaking it with your fingers.
Bake for 40 minutes.