01 -  Whisk together all-purpose flour, granulated sugar, baking powder, and salt in a medium mixing bowl. In a separate bowl, beat whole milk, large eggs, melted butter, and vanilla extract until smooth. Combine wet and dry mixtures, whisking until just blended. 
 02 -  Preheat the waffle iron. Pour batter onto the heated plates and cook until waffles are golden brown. Prepare 8 waffles to yield 4 sandwiches. 
 03 -  Melt unsalted butter in a large skillet over medium heat. Add apple slices, brown sugar, ground cinnamon, and a pinch of salt. Sauté, stirring frequently, until apples soften and become caramelized, around 8 to 10 minutes. Remove skillet from heat. 
 04 -  Heat granulated sugar and water in a small saucepan over medium heat until mixture turns a deep amber color, without stirring, about 5 to 7 minutes. Remove from heat and carefully whisk in heavy cream and butter; mixture will bubble vigorously. Stir in sea salt if desired and allow to cool slightly. 
 05 -  Arrange half the waffles on a serving platter. Spoon caramelized apples over each and drizzle with warm caramel sauce. Top with remaining waffles to form sandwiches and serve immediately, with extra caramel sauce if desired.