Special equipment: Dough cutter, 9-by-13-inch baking dish
For the dough: Preheat the oven to 375 degrees F. Lightly butter the sides and bottom of a 9-by-13-inch baking dish.
Using a dough cutter, combine the flour, baking powder, baking soda, salt and buttermilk powder if using, in a large bowl. Cut in the butter using your fingers or a dough cutter until the mixture resembles coarse meal. Add the buttermilk or 1 1/2 cups cold water if using buttermilk powder, and gently mix until the mixture just begins to come together.
Scrape the dough onto a lightly floured counter. Using a rolling pin, roll the dough into a 16-by-12-inch rectangle about 1/4-inch thick. Position the dough so the short side is facing you.
For the filling: Brush the surface of the dough with the softened butter. Combine the sugar, brown sugar and cinnamon in a small bowl. Spread the mixture evenly over the butter, leaving a 1/2-inch border. Spread the chopped pecans over the sugar mixture.
Fold 1/4-inch of the short side in. Roll the dough up tightly into a log starting on the shorter side. Using your hands to apply pressure, roll the log back and forth a few times to seal the dough. Slice 1/4-inch of dough off each side to even the ends, then cut the log into 8 equal pieces.
For the topping: Combine the butter and 1/4 cup water in a medium nonstick saucepan over high heat and cook until the butter is melted. Whisk in the brown sugar and cook until smooth and bubbly. Reduce the heat to low, whisk in the heavy cream and cook for 1 minute. Add the salt, then remove from the heat and keep warm.
Pour the warm topping into the prepared baking dish. Scatter some additional chopped pecans evenly over the surface. Place the buns in the baking dish, cut-side down. Bake until puffed and golden brown, 25 to 35 minutes.
Remove from the oven and let cool on a baking rack for 5 minutes. Carefully invert onto a platter.