Preheat oven 450 degrees. The shortening should be at least chilled and butter should be frozen.
In a large bowl whisk together dry ingredients: flour, baking powder, baking soda, salt, sugar until thoroughly blended.
Cut the shortening into small pieces. Add it to the bowl with your flour blend and work the butter into the flour with your hands or a fork. Just squeeze the ingredients between your fingers to break up the butter until you achieve a crumbly consistency. Use cold hands and try work quickly, so the butter doesn't melt.
Grate the frozen butter with a box grater, then fold it into the dry ingredients.
Create a well in the middle of the bowl. Pour in chilled buttermilk in the center of the well the around the edges of the bowl. Use the rubber spatula to fold in the ingredients together trying to incorporate the wet with the dry. Continue to fold until the dough pulls away from the edges of the bowl.
Lightly flour your surface and hands then knead dough 3-4 times until the ingredients come together. Use a floured rolling pin and roll dough into a rectangular-ish shape. Fold twice and roll out again. Repeat roll and fold 3 more times and then cut into 12-15 rounds using 2-inch cookie cutter. You can repeat the roll and fold process if you have any excess dough.
Place biscuit onto a prepared baking sheet or baking mat. Melt the remaining 2 tablespoons of shortening and brush the tops of each biscuit. Place into oven and bake for 12-15 minutes, or until golden brown.
Hot Buttered Rum Sauce
In a medium sauce pan on medium high heat, add brown sugar, vanilla, ground cinnamon, nutmeg, allspice and 1/2 cup of rum. Continue to stir until all sugar is dissolved and mixture comes to a strong boil.
Reduce heat to medium, allow mixture to reduce and thicken and stir occasionally.
Once the mixture reaches desired thickness (approximately 10 minutes) stir in butter a tablespoon at at time. Then stir in salt and remaining 2 tablespoons of rum. Serve.