Melt 2 tablespoons unsalted butter in a medium skillet over medium heat. Add 4 coarsely crushed graham cracker sheets, 1 tablespoon granulated sugar, and 1 pinch kosher salt. Cook, stirring often, until toasted and fragrant, 3 to 5 minutes. Transfer to a plate and let cool.

Beat 1 cup of the cold heavy cream in a stand mixer with the whisk attachment on medium-high speed until soft peaks form, 1 to 1 ½ minutes. Transfer to a large bowl.

Beat 8 ounces room-temperature cream cheese and 1 (14-ounce) can sweetened condensed milk in the now-empty bowl (no need to clean) with the paddle attachment on medium speed until smooth and creamy, about 1 minute. Beat in 1 teaspoon finely grated lime zest and ½ cup lime juice on medium speed until smooth and combined, scraping down the sides of the bowl as needed, about 1 minute total.

Gently fold in the whipped cream with a flexible spatula just until combined.

Place 2 tablespoons of the graham cracker crumbs into each of 6 (8-ounce) glasses, jars, or ramekins; reserve the remaining for garnish. Divide the mousse between the glasses (about heaping ¾ cup each). Alternatively, make smaller servings by filling 12 (4-ounce) glasses with 1 tablespoon crumbs and heaping ⅓ cup mousse each. Refrigerate for at least 2 hours and up to 3 days.

Just before serving, beat the remaining ⅔ cup cold heavy cream with ⅓ cup sour cream together in a stand mixer with the whisk attachment on medium-high speed just until soft peaks form, about 2 minutes. Divide between the glasses. Garnish with more finely grated lime zest and the reserved graham cracker crumbs.