Spray a 9"x13" baking pan,with non-stick cooking spray. Line pan with a sheet of parchment paper (leaving long edges for easy removal later) and spray parchment paper.
Whisk the first 5 ingredients together in a medium bowl until combined. Place into a food processor bowl and add butter. Pulse mixture until moist and has texture of pebbles. Stir in caramel chips. Alternately: cut the butter into small bits, add to bowl, and use your fingers to mix. Set aside.
Whisk brown sugar and the next 6 ingredients together in a small bowl until combined. Set aside.
Using a mixer (stand or hand), beat the butter and sugars together in a large bowl until they are well-combined, 3-4 minutes. Beat in the vanilla and one egg at a time until light and fluffy.
In a medium bowl whisk the flour, baking powder, and salt together, set aside
Measure buttermilk into a glass measuring cup and add sour cream, stir until blended. and set aside.
Beat the dry mixture into the butter sugar mixture in thirds, alternating with the wet mixture, until just combined. Gently fold in the salted caramel chips.
Scoop half of the batter into the prepared pan and smooth evenly.
Scatter the filling mixture evenly over the batter.
Add the remaining batter on top of the filling in dollops, then using an off-set spatula or knife, gently spread it over the filling. Run a skewer, through the batter a few times to swirl the filling a bit.
Scatter the streusel over the top of the entire cake.
Place pan into the preheated oven and bake for 60 minutes, or until the streusel is golden and a toothpick or skewer inserted in the center should come out clean.
Allow to cool for 30 minutes before cutting,using the parchment paper "handles" to lift it from the pan.
Store completely cooled cake wrapped, at cool room temperature.