Generously butter bottom and sides of 10in cast iron skillet.
Cream butter and sugar. Add eggs one at a time. Incorporating the first before adding the second.
In a separate bowl, mix together flour, baking powder and salt.
Add 1/3 of dry mixture. Completely incorporate without over mixing. Add 1/3 buttermilk. Incorporate completely again without over mixing. Repeat until all dry ingredients and buttermilk have been incorporated.
Add vanilla and mix just until incorporated evenly.
Briefly heat skillet in oven until butter is beginning to melt. Remove from oven and add 1/2 of batter. There will be a nice sizzle (similar to making corn bread). Distribute blackberry jam over the batter, staying 1/2 inch from the edges. Add rest of cake batter on top.
Bake at 375 degrees Fahrenheit for 25-30 minutes. Cakes should be golden brown. Remove from oven. Let cool for 5 minutes. Drizzle 1/2 of orange simple syrup over top. Let soak and cool completely before dusting with powdered sugar and serving.
Orange Simple Syrup
Before baking cake, add simple syrup ingredients to small sauce pan and bring to a simmer while stirring. Let simmer for 20 minutes while you make your cake batter. After 20 minutes, remove from heat and allow to cool before drizzling over finished cake.
The orange and cinnamon infused simple syrup give this cake a yummy seasonal flair. Leave out the cinnamon and use the fruit preserves of your choice for variety!