Preheat oven to 350 degrees.  Spray a 9" pie plate with nonstick spray.  Crush pretzels in a food processor {or roll in a Ziploc bag}, then add melted butter and sugar until combined.  Press evenly into pie plate and bake for 8-10 minutes.  Cool completely. 

In a mixing bowl add 3/4 cup of heavy whipping cream and beat with an electric mixer until stiff peaks form.  Set aside.  In a large mixing bowl add cream cheese and beat with an electric mixer until smooth.  Once smooth, add sweetened condensed milk, lime juice and lime zest, mix until combined.  Fold in whipped heavy cream until incorporated.  Pour mixture into cooled crust and spread into an even layer.  Refrigerate for at least an hour, or until set.

Sweetened Whipped Cream:  Add whipped cream to a mixing bowl with powdered sugar, vanilla and beat with an electric mixer until stiff peaks form.  Pipe onto pie and garnish with lime slices, if desired.