For the cake: Preheat the oven to 350 degrees F. Melt together the butter and brown sugar in a 10-inch cast-iron skillet on medium heat. When the sugar and butter have melted together stir in the blackberries, pineapple, thyme leaves and salt. Cook, stirring occasionally, until the brown sugar begins to bubble, about 3 minutes. Once this happens turn the heat off.
Prepare the cake mix as directed on the package and pour over the fruit. Bake until the cake is slightly golden brown, 30 to 35 minutes.
Meanwhile, make the creme fraiche: Whisk the creme fraiche, sugar, vanilla bean seeds, lemon zest and salt in a medium bowl to stiff peaks.
To serve: Serve slices of cake warm or at room temperature with dollops of vanilla creme fraiche.