The buttermilk brings a richness and moistness to this cake that's absolutely unreal. When paired with a salted caramel buttercream and a salted caramel drizzle, you've got some real magic on your hands!
In a medium-sized bowl, mix together the flour, baking soda, salt, baking powder, cinnamon, and allspice until incorporated. Set aside.
Evenly distribute the batter between 3 (8-inch) well-greased cake pans whose bottoms are lined with an 8-inch circle of parchment paper. Place the pans in the oven and bake until a toothpick inserted into the center of the cake comes out clean, about 35 minutes. Remove the pans from the oven and allow to cool for 10 minutes before removing the cakes from the pans and allowing them to cool completely on a wire rack.
While the cake is baking and cooling, you can prepare the caramel. Bring the granulated sugar, water, and minced pear to a boil over medium high heat and continue boiling until the mixture turns a light caramel color and smells pleasantly toasty, only stirring once every four minutes. This could take anywhere between 8 to 16 minutes, the speed will depend upon the heat intensity of your stovetop.
Remove the pan from the heat and quickly stir in the heavy cream, brown sugar, and butter until incorporated. Be careful as the mixture will spit and hiss a bit. If the sugar begins to clump up when you're stirring do not fret, just stir as best as you can for about 30 seconds and then put the pot back on the heat and bring it back to a boil again over medium-low heat.
Once it is boiling again, stir until the sugar chunk dissolves and the mixture is smooth. Once it smoothes out, stir it every minutes or so and allow the mixture to continue simmering and thickening. When the caramel holds its shape (rather than immediately disintegrating) when you put a drop of it in a cold glass of water, it's thickened enough. Remove from the pot from the heat, stir in the salt, and set aside to cool completely.
Once the caramel has cooled, you can prepare the buttercream. In the bowl of an electric stand mixer fitted with the paddle attachment, beat the butter, powdered sugar, and caramel together at medium-low speed until relatively smooth. Add the whipping cream, whiskey, vanilla, and salt and increase the speed to medium, beating until the buttercream is light and fluffy, about 1 minute.