Preheat oven to 350 degrees. Line a baking sheet with parchment.
First, make pavlova: Beat egg whites and pinch salt until soft peaks form. Gradually add sugar, one spoonful at a time, and beat until shiny and stiff. Add apple cider vinegar and vanilla and beat just until blended. Sift cornstarch over and fold in. Heap meringue on parchment, forming a 9-inch circle, smoothing the top and edges. Place in oven, lower temperature to 300 degrees, and bake 1 hour. Turn off heat and leave meringue in oven to cool.
Meanwhile, make pastry cream: Heat milk until small bubbles form around the edges. Beat egg yolks and sugar until thick and pale yellow. Add flour and mix thoroughly. Gradually add hot milk, a few drops at a time, beating constantly. Heat mixture over moderate heat, whisking constantly, until it thickens. Lower heat and cook 2-3 minutes more, stirring constantly. Remove from hear and beat in butter, rum and salt. Chill.
For caramel sauce: Combine water, sugar and salt in a heavy saucepan. Stir over moderate heat until it starts to boil. Lower heat and cook without stirring for 5-6 minutes, or until it turns a medium amber. Watch carefully at this point – it should take on a rich amber color but not turn too dark. Immediately add cream – it will bubble and splutter, so be careful – and reduce heat, stirring constantly for about 3 minutes, until it forms a thick sauce. Remove from heat and add vanilla. Cool to room temperature.
To assemble: Slice bananas and toss gently with lemon juice. Beat whipping cream until soft (not stuff) peaks form.
Place pavlova on flat serving plate. Spread pastry cream atop (some will spill off the sides. Place banana slices on pastry cream. Heap whipped cream on top of bananas. Use a spoon to drizzle cooled caramel on top. Cut into wedges and serve immediately. Refrigerate leftovers.