Combine the sugar and water in a medium saucepan and bring to a boil over medium-low heat; continue to cook, without stirring, until the sugar has dissolved (about 4 min.).
Increase the heat to medium-high, then cook until the caramel has turned to a deep golden color (about 6 to 7 min.).
Remove from heat. Add the heavy cream and whisk to incorporate, being careful as the heavy cream will bubble heavily.
Return the saucepan to the heat. Add the butter and salt; stir. Cook for another minute.
Turn off the heat and let the caramel sauce cool down before using. (The caramel will get thicker after it cools down but it won’t harden at room temperature.)
Preheat the oven to 400°F.
Line a 2-inch-deep 8x8-inch pan with parchment paper.
In a small bowl, combine the pretzel crumbs, sugar and butter.
Press the mixture into the bottom of the pan and bake for 12 min. or until golden brown.
Reduce the oven temperature to 325°F.
In a medium bowl, beat the cream cheese and sugar until smooth.
Add the eggs, one at time, and mix until just blended.
Stir in the vanilla and flour just until combined.
Pour the cream cheese batter over the crust. Add ½ cup of the salted caramel sauce and swirl it into the cream cheese batter. Bake for 30 min. or until the center of cheesecake is set.
Let the cheesecake cool completely at room temperature.
Pour the remaining 1 cup salted caramel sauce over the cooled cheesecake, then refrigerate overnight (or at least 4 hours).
Before serving, decorate with whole pretzels and salted caramel popcorn.