In a large bowl, whisk together the 3 cups flour, salt and baking powder.
Whisk in the granulated sugar.
Add in the vanilla, eggs, melted butter and milk and mix everything together just until combined. Set aside.
In a medium bowl, mix together all of the filling ingredients: melted butter, brown sugar, flour, cinnamon and milk. Set aside. NOTE: if using the optional pecan filling in the Notes below use those ingredients.
Divide the cake batter in half. Pour half of it into the bottom of the prepared pan. Smooth for even thickness.
Divide the filling in half. Spoon half of it over the cake batter. Using a knife or toothpick, swirl the filling around to spread it out.
Pour the remaining cake batter on top and spread it out to the edges.
Spoon the remaining filling on top of the cake and use a knife or toothpick to swirl the filling around.
Bake for 30-35 minutes until a toothpick inserted into the middle comes out clean.