Pour the sweetened condensed milk into a saucepan. Set over medium-to-medium-low heat and stir continuously, until the condensed milk has gone from white to light caramel color, about 20 minutes.
Preheat oven to 350˚ F and grease a 13x9 inch pan. Set aside.
To make the bars: In a large bowl, beat together the butter and sugar with a handheld or stand mixer on medium-high until light and fluffy, around 5 minutes.
Add the sour cream, egg, and vanilla extract and continue to beat until thoroughly combined.
Beat in the flour, baking powder, and salt until just combined, then press into the bottom of the greased pan.
In the same bowl as the dough, add the cream cheese, sugar, egg, vanilla, cinnamon, and salt. Beat until well mixed, then pour out on top of the cookie dough and spread evenly. Drizzle with dulce de leche and dark chocolate, swirl around the cheesecake layer, then bake for 25-30 minutes. Allow to cool completely, then cut and serve.