In a medium bowl, combine crushed vanilla wafers, melted butter, and granulated sugar. Press firmly into the bottom of a 9-inch springform pan. Freeze for 15 minutes.
Spread the softened soft serve ice cream evenly over the crust. Smooth the top with a spatula. Freeze for at least 4 hours or until firm.
In a large bowl, whip heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
Remove the cake from the springform pan. Spread the whipped cream over the top. Garnish with rainbow sprinkles if desired. Serve immediately or return to freezer until ready to serve.
Tips
For easier slicing, dip a knife in hot water before cutting each slice.
Store leftover cake in the freezer for up to 1 week.