Grease 3 8"inch cake pans. Preheat the oven to 325°F/162°C
In a large bowl, sift the flour. In a measuring jug, mix the apple cider and cider vinegar together.
Add butter, coconut oil, sugars, spices, baking powder, baking soda and salt in the bowl of a stand mixer. Combine using the paddle attachment, first on low and then increasing the speed to medium until fluffy, for about 5 minutes.
While the mixer is running, add the egg whites and then the vanilla extract. Reduce the speed to low and add a third of the flour, then a third of the apple cider. Repeat twice more. Once the mixture is completely smooth, pour into the three cake pans.
Bake for 40 minutes, until the cake is brown around the edges and a toothpick would come out with just a few crumbs.
Pie Crumb
Heat oven to 355°F/180°C and line a baking sheet with parchment paper.
Combine the flour, sugar and salt in a mixer with a paddle attachment on low until mixed. Add butter and water and mix on low, it should start forming clusters.
Spread the clusters on the tray and bake for 25 minutes, until golden brown. Let cool completely.
Salted Caramel
Put the sugar in a medium saucepan and stir constantly over a medium heat. The sugar will start forming clumps and then begin to melt. Once it turns a deep amber colour, take off the heat and add all the butter. Be careful as it may sputter! Then add the cream and the salt. Let it cool completely.
If you make this ahead of time, keep it in the fridge.
Swiss Meringue Buttercream
Fill a medium saucepan with about an inch of water, bring to a gentle boil.
In a stand mixer bowl, stir the egg whites, sugar and salt until combined. Place the bowl over the saucepan. Make sure you're constantly whisking the sugar mixture until it reaches a temperature of 160°F.
Remove the bowl from heat and place it in your stand mixer, with a whisk attachment. Whisk on medium high until you have stiff, glossy peaks, about 8-10 minutes. Touch the bowl, at this point it should have cooled down.
Reduce speed to low and add the butter, a tablespoon at a time.
Fillings & Frosting
Take 400g of the buttercream out and put into a large bowl. Add 120g of salted caramel and mix well with a spatula.
Combine half of the pie crumb, milk and salt in a blender. Puree until smooth. Add this to the rest of the buttercream and fold using a flexible spatula.