1. Preheat oven to 350 degrees. Butter the sides of a 10-inch springform pan. Wrap exterior of pan (including base) in a double layer of foil. Melt 6 tablespoons butter in a small saucepan over medium heat. Place crushed wafer cookies and 3 tablespoons sugar in a large bowl; stir until well combined. Pour melted butter over cookie mixture and mix until evenly moistened. Press mixture evenly into the bottom of the prepared pan.

2. Place pan on a baking sheet. Bake until crust is set, 10 to 12 minutes. Transfer pan to a wire rack to cool completely.

3. In the bowl of an electric mixer fitted with the paddle attachment, beat the cream cheese on medium speed until fluffy, about 3 minutes, scraping down sides as needed. In a large bowl, whisk together remaining 2 1/4 cups sugar and flour. With mixer on low speed, gradually add sugar mixture to cream cheese; mix until smooth. Add sour cream and vanilla; mix until smooth. Add eggs, one at a time, beating until just combined; do not overmix.

4. Pour cream cheese filling into prepared pan. Set pan inside a large, shallow roasting pan. Carefully ladle boiling water into roasting pan to reach halfway up sides of springform pan. Bake 45 minutes; reduce oven temperature to 325 degrees. Continue baking until cake is set but still slightly wobbly in the center, about 30 minutes more. Turn off oven; leave cake in oven with door slightly ajar, 1 hour. Transfer pan to a wire rack; let cake cool completely. Refrigerate, uncovered, at least 6 hours or overnight. Before unmolding, run a knife around the edge of the cake.

5. Slice the top and bottom of 1 orange with a paring knife. Set one of the flat sides on your work surface. From top to bottom, following the curve of the fruit, cut away peel and white pith. Trim off any remaining pith. Holding the orange over a bowl, cut along both sides of each segment, staying close to the membrane, to release. Repeat with remaining oranges. Pour in liqueur and toss to combine. Serve over cheesecake.