Step 1

Place one rack on the bottom of the oven and one in the middle, then preheat oven to 325 degrees. Butter a 13 x 9 x 2 inch baking pan and line the bottom with parchment paper. Butter the paper as well and sprinkle evenly with the turbinado sugar.

Step 2

Beat the butter and sugars until fluffy, then beat in eggs one at a time. Add vanilla and mix well. Pour the flour, baking powder, soda, salt, and cinnamon into a strainer and sift into the wet ingredients. Mix on low just until incorporated. Scrape down the sides and add the buttermilk and rum and mix until smooth.

Step 3

Add the batter in scoops across the pan so that it is fairly evenly distributed, then gently spread into a single layer taking care not to disturb the turbinado sugar. Place baking pan on the bottom rack and bake for 20 minutes, then move it to the middle rack and continue baking until a toothpick inserted into the center of the cake comes out clean, about 12-15 more minutes.

Step 4

Remove the cal from the oven and immediately heat the ingredients for the buttered rum in a small saucepan over medium heat just into the sugar dissolves and the butter melts. Punch holes in the top of the cake with a skewer, and then pour the buttered rum mixture evenly over the top, making sure to cover the entire cake.

Step 5

Let cake cool completely at room temperature before carefully inverting it onto a flat platter. Store covered at room temperature.