Reduce the champagne: Pour 1 cup champagne into a small saucepan. Simmer over medium heat until reduced to about 1/3 cup, 12–15 minutes. Cool completely. This concentrates the flavor without excess liquid.

Prep the pan: Heat the oven to 325°F (163°C). Line a 9x13-inch baking pan with parchment, leaving overhang on two sides for easy lifting.

Make the crust: Combine 2 cups crumbs, a pinch of salt, and 6 tbsp melted butter. Stir until the texture feels like damp sand. Press firmly into the lined pan in an even layer. Bake 8–10 minutes until lightly golden. Cool while you make the filling.

Beat the cream cheese: In a large bowl, beat the softened cream cheese on medium speed until smooth and fluffy, about 2 minutes. Scrape the bowl often to prevent lumps.

Add sugar and flavorings: Beat in 3/4 cup sugar, the cooled champagne reduction, 2 tsp vanilla, 1/8 tsp salt, and lemon zest if using. Mix until smooth.

Blend in sour cream and eggs: Beat in the sour cream just to combine. Add the eggs one at a time on low speed. Mix only until incorporated. Do not overbeat or you’ll add too much air.

Fill and bake: Pour the batter over the crust and smooth the top. Bake at 325°F for 28–35 minutes. The edges should be set and the center should wobble slightly when you nudge the pan.

Cool gradually: Turn off the oven, crack the door, and let the bars sit for 10 minutes. Move the pan to a rack and cool to room temperature. This helps prevent cracks.

Make the champagne topping: Sprinkle 1 1/2 tsp gelatin over 2 tbsp cold water in a small bowl; let bloom 5 minutes. In a saucepan, warm 1/2 cup champagne with 2 tbsp sugar over low heat until the sugar dissolves and the mixture is hot but not boiling. Stir in the bloomed gelatin until fully dissolved. Cool to lukewarm.

Add the topping: Pour the lukewarm champagne mixture over the cooled cheesecake layer, tilting the pan for even coverage. Chill uncovered for 30 minutes, then cover and refrigerate at least 4 hours, preferably overnight, until fully set.

Slice and serve: Use the parchment to lift the slab onto a cutting board. Warm a sharp knife under hot water, wipe dry, and cut into 20–24 bars. Garnish with fresh berries, a dusting of white chocolate, or a touch of edible glitter for a celebratory look.