Preheat the oven to 350 degrees F. Cook the bacon in a large skillet over medium heat, turning occasionally, until crisp, 9 to 11 minutes; remove to paper towels. Finely chop and set aside.
Pour off all but 3 tablespoons drippings from the skillet. Add 4 tablespoons butter and melt over low heat. Pour into a 9-inch-round cake pan, tilting the pan to coat the bottom. Sprinkle the brown sugar evenly over the butter mixture.
Place a pineapple ring in the middle of the pan, then arrange the remaining rings around the outside, overlapping them slightly if necessary. Sprinkle about 3 tablespoons of the chopped bacon in the middle of the rings and between them.
Whisk the flour, baking powder, baking soda and salt in a medium bowl. Beat the remaining 1 stick butter and the granulated sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 2 minutes. Beat in the eggs, one at a time, then beat in the vanilla. Reduce the mixer speed to low and beat in the flour mixture in 3 additions, alternating with the buttermilk, beginning and ending with the flour mixture. Increase the mixer speed to high and beat until the batter is smooth, about 15 more seconds. Fold in the remaining chopped bacon.
Spread the batter over the pineapple in the pan. Bake until a toothpick inserted into the center comes out clean, about 1 hour. Transfer to a rack and let cool 15 minutes, then run a knife around the edge and invert onto a platter. Let cool at least 1 hour before serving.