Preheat oven to 350 degrees F.
Using one tablespoon of butter, grease an eight-inch springform pan with parchment paper lining the bottom; set aside.
Toss squash slices with 1 tablespoon butter (melted), 1/4 cup sugar, cinnamon and 1 teaspoon salt. Roast on a baking sheet until tender, approximately 20-25 minutes.
Increase oven heat to 375 degrees F.
Heat lemon juice, remaining sugar, and 1/4 cup water in a saucepan. Swirl pan while boiling (don’t stir) until liquid deepens to a golden-brown color (for approximately 5-10 minutes). Remove from heat and add remaining four tablespoons of butter, whisking until fully incorporated. Add vanilla and remaining salt; whisk until smooth.
Pour caramel into the prepared springform pan, coating entire bottom surface. Arrange squash slices in a decorative pattern at the bottom of pan/on top of caramel. Fit pastry round on top of all, tucking edges under on all sides.
Bake for 15 minutes, then reduce heat to 350 degrees F and continue baking for another 45 minutes. Cool, then remove springform ring and carefully flip onto wire rack. Slice and serve.