Combine graham cracker crumbs and granulated sugar in a mixing bowl or right in a 9x13 baking dish.
Melt butter in a heat-proof bowl in the microwave. Pour melted butter into the graham cracker mixture and stir until evenly combined.
Press the graham cracker mixture evenly across the bottom of the baking dish. Place the baking dish in the freezer to firm up the crust while making the cheesecake mixture.
In a medium mixing bowl, beat together cream cheese, 1¼ cups of powdered sugar, vanilla extract, and sour cream until thoroughly combined.
Pour 3 cups of heavy whipping cream into a chilled mixing bowl. Beat on medium-high speed (either with a stand mixer whisk attachment or with a handheld mixer). When the whipping cream thickens to soft peaks, add in the remaining ½ cup powdered sugar (2 to 3 minutes).
Continue beating until whipped cream forms stiff peaks. Set aside about 2 cups of the whipped cream for decorating the top of the cheesecake.
Use a spatula to gently fold the remaining whipped cream (about 4 cups) into the cream cheese mixture.
Spread the cheesecake filling evenly over the chilled graham cracker crust. Use an offset spatula to smooth the top of the cheesecake.
Place the cheesecake in the refrigerator for at least 4 hours or in the freezer for at least 2 hours to set.
Transfer reserved whipped cream to a piping bag fit with Tip #1M.
Pipe 12 dollops of whipped cream onto the cheesecake, evenly spaced to separate the pan into 12 even pieces.