Preheat oven to 350F. Grease and flour three 6″ cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, spices, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and brown sugar on medium-high until pale and fluffy (approx 3 mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla. Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk), fully incorporating after each addition.
Bake for 30-35 mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10 mins then turn out onto wire rack. Allow to cool completely.
CARAMEL APPLE FILLING:
Dice apples & toss with lemon juice in medium – large bowl.
In separate bowl, combine brown sugar, cinnamon & cornstarch. Whisk to combine. Set aside.
Transfer apples to a small saucepan, add water & heat over med-high heat about 8 minutes.
Some of the apples will break down into a thick liquid like sauce but you will still have some apple chunks.
Add your sugar mixture & whisk to combine and incorporate well.
Heat & stir constantly until mixture thickens, about 2 minutes. Remove from heat & let cool. Make sure the apple mixture is completely cool before frosting the cake.
BROWNED BUTTER CREAM CHEESE FROSTING:
Slice the butter up into pieces and place in a light-colored skillet. (Light colored helps you determine when the butter begins browning.)
Melt the butter over medium heat, stirring occasionally. Once melted, the butter will begin to foam. Keep stirring occasionally.
After 5-8 minutes, the butter will begin browning– you’ll notice lightly browned specks begin to form at the bottom of the pan and it will have a nutty aroma. Once browned, remove from heat, and immediately pour into a heat-proof bowl. Refrigerate until solid.
In a large bowl using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the solid brown butter on high speed until creamed.
Add the cream cheese and beat on high speed until combined, smooth, and creamy.
Add 4 cups confectioners’ sugar, vanilla, and salt. Beat on low speed for 30 seconds, then switch to high speed and beat for 2 minutes. If you want the frosting a little thicker, add the extra ¼ cup of confectioners sugar.
ASSEMBLE THE CAKE:
Place one layer of cake on a cake stand or serving plate. Pipe a ring of cream cheese frosting around the edge, then spoon in ⅓ of the cooled apple filling. Repeat with remaining layers and apply a thin coat of frosting all over the cake.
Chill for 20 mins.Spoon the remaining apple filling near the edge of the cake to create a drip (make sure the cake is cooled when you do this for the best results). Use the remaining frosting rustic drops and swoops around the outer edge of the cake to hold in the apple filling.