Make the crust. Preheat your oven to 375°F with the rack positioned in the center. Then, crush the graham crackers into fine crumbs using a food processor. Pulse until smooth. Next, combine the graham cracker crumbs, melted butter, sugar, and cinnamon in a mixing bowl. Stir until the mixture resembles wet sand. After that, firmly press the crumb mixture evenly into the bottom and sides of a 9-inch pie plate to form the crust. Bake the crust for 7 minutes, making sure not to overbake as this can make the crust too hard to slice. Allow the crust to cool completely at room temperature before adding the filling.
Make the filling. In a large mixing bowl, whisk the sweetened condensed milk, lemon juice, lemon zest, sour cream, and egg yolks until smooth and well combined. Lower the oven temperature to 350°F.
Assemble and bake. Pour the lemon filling into the cooled pie crust, using a spatula to spread it evenly. Bake the pie for 10 minutes to set the filling. When done, it will remain pale and jiggle slightly in the center. Remove the pie from the oven and allow it to cool at room temperature for about 1 hour. Once cooled, transfer the pie to the refrigerator and chill for at least 8 hours or overnight for the best texture.
Garnish and serve. Before serving, top the chilled pie with freshly whipped cream and garnish with thin slices of lemon for a bright, decorative touch. Slice and enjoy your creamy, tart lemon pie cold.