Preheat oven to 350°F (180°C). Liberally grease two 8" round cake pans with vegetable shortening, and/or spray with baking spray.

Combine all of the dry ingredients in a large mixing bowl, set aside.

Carefully separate eggs, placing yolks in the dry ingredients bowl along with buttermilk, shredded apple, melted butter, and vanilla. Whisk until smooth and well combined.

In a – very clean – mixing bowl or stand mixer, whip the egg whites on high speed, until stiff peaks form.

Gently fold whipped egg whites into the main batter, until all of the “white” disappears.

Divide batter among prepared cake pans. Bake until golden and knife inserted into center of batter comes out clean and cake springs back - about 45-60 minutes.

Allow to cool 10-15 minutes before turning cakes out onto baking rack to cool fully.

Ideally, allow to cool to room temperature, wrap tightly with plastic wrap. Allow cake to sit overnight, make frosting & ice cake the following day.

In a medium saucepan, melt butter over medium heat.

Add brown sugar, cinnamon, nutmeg, and salt , stirring until well combined.

Add apple slices, and cook until tender. The juices will come out of the apples, form a syrup, and then eventually mostly cook off to a thick glaze.

Remove from heat, allow to cool to room temperature.

Note: I like to drain the apples off the syrup, and put the syrup in a pastry bag to be drizzled over the final cake, later.

Mix egg whites and brown sugar in a very clean metal mixing bowl, and place over a pot of simmering water on your stove top.

Whisk occasionally until it hits 160°F (72°C) on a candy thermometer.

Move egg mixture to your stand mixer and whip on high (using the whisk attachment) until stiff peaks form, and mixture is relatively cool.

While waiting, cut up the butter into chunks.

When meringue has reached the stiff peaks stage, change the mixer over to the paddle attachment.

With the mixer on LOW speed, add the butter a chunk at a time.

Add flavoring, continuing to mix until fully incorporated.

Once butter and flavoring are incorporated into the mix, turn speed back up to high and whip until you have a smooth buttercream.

It will go through some weird stages before this point – soupy, maybe curdled. Don’t worry! It will come together!

Level your cakes by cutting the rounded “dome” off the top of each cake round.

Smear a small amount of frosting on your cake board or cake drum.

Take one round of leveled cake, and center it on your board or drum. The frosting will act as a glue, lightly holding the cake in place.

Spread a little bit of the frosting on the cake round, smoothing to an even layer.

Spoon some brown sugar Swiss meringue buttercream into a pastry bag, and cut the tip off - aim for about ½" diameter hole.

Use the decorating bag to evenly pipe an unbroken border along the top edge of the bottom layer. This border should be positioned slightly to the inside of the edge.

Spread the cooled apple filling inside this “dam”, top with second layer of cake. For easier handling, you can wrap the cake in plastic and chill for an hour at this point.

Using an offset spatula (frosting knife), spread a thin coat of frosting over the top and sides of your cake. Fill in any gaps as necessary, and smooth everything as you go.

This step will very likely stir up some crumbs as you go - that’s perfectly ok! This is why we call it a “crumb coat”, after all!

When your cake is completely coated with frosting, return it to the fridge to chill for another 30 minutes or so.

Clean off your spatula, use it to apply a generous layer of frosting to your chilled, crumb coated cake.

I like to frost the sides first, before scooping a generous amount of frosting onto the top of the cake. I spread it out from the center of the cake, towards the edges. Once the top and sides of frosting meet, I tidy it all up with a few more swipes of my spatula.

Decorate the cake with extra frosting, the leftover sauce, and/or fresh apple slices. (Soak the apple slices in lemon juice for a few minutes before using!)