Heat the oven to 350°F (175°C). Grease and line a 9x13 pan with parchment for easy removal.
Spread the pecans on a sheet pan and toast for 7–9 minutes until fragrant. Cool, then chop 1 1/2 cups for the batter; reserve 1 cup halves for topping.
In a large bowl, whisk flour, baking powder, baking soda, and salt until combined.
In another bowl, whisk melted butter, oil, dark brown sugar, and granulated sugar until glossy and slightly thick, about 30 seconds.
Whisk in eggs one at a time, then vanilla. Stream in buttermilk and whisk until smooth.
Add the wet ingredients to the dry and whisk just until you no longer see dry flour. Fold in the chopped toasted pecans.
Scrape the batter into the prepared pan and smooth the top. Bake for 26–32 minutes until the center springs back and a toothpick comes out with a few moist crumbs.
Cool the cake in the pan on a rack for 15 minutes. Poke holes all over with a skewer to help the caramel seep in.
In a medium saucepan over medium heat, melt butter. Stir in dark brown sugar and maple or corn syrup. Cook, stirring, until the mixture looks glossy and cohesive, about 2 minutes.
Slowly pour in the heavy cream (it will bubble). Stir constantly and bring to a gentle simmer. Cook for 3–5 minutes until slightly thickened.
Remove from heat. Stir in vanilla and salt. The caramel should coat the back of a spoon but still pour easily.
Pour the warm caramel evenly over the warm cake, letting it sink into the holes.
Scatter the pecan halves on top and gently press so they adhere.
Optional: Sprinkle with a pinch of flaky sea salt for contrast.
Cool for at least 30 minutes before slicing. The caramel sets to a gooey sheen as it rests.
