Preheat the oven to 350°. Butter and flour two 8-by-2-inch round cake pans. In a large bowl, sift the flour together with the baking powder, spices, and, salt.
In a stand mixer or with a hand mixer, beat the butter until creamy. Add the brown sugar and molasses and beat 4 minutes until light and fluffy, scraping down the bowl as needed.
Add the eggs, 1 at a time, beating well after each addition. Mix in the vanilla and bourbon.
Beat in the flour mixture on low speed in 3 batches, alternating with the buttermilk, until the batter is just combined.
Divide the batter evenly between the prepared pans. Bake for 35 to 40 minutes, or until a toothpick inserted in the center comes out clean.
Let the cakes cool for 10 minutes, then turn each out onto a cooling rack and let cool completely. Wrap each cake in plastic and store at room temperature overnight to develop the spice flavors.
When ready to decorate, use a long serrated knife to cut each cake layer in half horizontally. Set on a work surface, cut side up.
Center 1 bottom cake layer on a cake board or plate. Spread 1/2 cup of the Caramel Cream Cheese Buttercream on top, then cover with another layer, cut side down.
Repeat with the remaining cake layers and buttercream. Crumb ice the cake, then refrigerate for 15 minutes. Completely cover the cake with the remaining buttercream.
Pour 1 cup of caramel sauce on top of the cake and use a spatula or knife to spread it to the edges, allowing it to drip down the sides. Decorate as desired with gingersnap cookies.
Bourbon Caramel Sauce
Heat sugar and water in a medium saucepan over medium heat, stirring to moisten all of the sugar. Let the sugar simmer until it begins to melt into a golden-colored liquid, stirring occasionally.
Once sugar is completely melted, immediately but carefully stir in the butter for 2-3 minutes until completely melted. The sauce will bubble rapidly.
Slowly drizzle in the heavy cream, continuing to stir. The mixture will once again bubble and splatter. After the cream is mixed in, allow the caramel to boil for about 1 minute, then whisk until smooth.
Remove from heat and whisk in bourbon, vanilla, and salt. Cool to room temperature, then chill for 15 minutes before using.
Caramel Cream Cheese Buttercream
In a mixing bowl, beat butter, cream cheese, and caramel sauce until well blended. Mix in powdered sugar, vanilla, and salt until fluffy.