Preheat oven to 350°F (180°C). Spray a 9½-inch fluted round removable-bottom tart pan with baking spray with flour.

In the work bowl of a food processor, combine graham cracker crumbs and sugar, pulse until well combined. Add melted butter; pulse until well combined and mixture holds together when pinched. Press graham cracker mixture into bottom and up sides of prepared pan.

Bake until crust is set and fragrant, 8 to 10 minutes. Let cool completely on a wire rack.

In the top of a double boiler, combine semisweet chocolate, 1 cup (240 grams) cold cream, and salt. Cook over simmering water, stirring frequently, until chocolate is melted, and mixture is smooth. Remove from heat; gradually whisk in ⅓ cup (80 grams) sparkling wine until smooth and well combined. Pour mixture into prepared crust. Refrigerate until semisweet chocolate mixture is set, at least 3 hours or up to overnight.

In the clean top of a double boiler, combine white chocolate and ⅓ cup (80 grams) cold cream. Cook over simmering water, stirring frequently, until chocolate is melted, and mixture is smooth. Transfer white chocolate mixture to the bowl of a stand mixer and let cool to room temperature (70°F/21°C).

Using the whisk attachment, beat cooled white chocolate mixture at medium speed; gradually add remaining 2 tablespoons (30 grams) sparkling wine, beating until well combined. Reduce mixer speed to medium-low; slowly add ⅓ cup (80 grams) cold cream, beating until smooth and stopping frequently to scrape sides of bowl. Increase mixer speed to medium; gradually add remaining ⅓ cup (80 grams) cold cream, beating until medium-stiff peaks form. (Whisk by hand toward end if needed; do not overmix.) Spread and swirl topping on tart as desired, being careful not to overhandle white chocolate mixture. Garnish with chocolate pearls and chocolate shavings, if desired. Refrigerate until topping is set, about 30 minutes. Remove from pan and serve immediately.