CRUST:
Preheat oven to 350°F. Wrap the outside of a 9-inch springform pan with heavy-duty aluminum foil. Grease the inside with butter.
Make crust: Pulse gingersnaps in a food processor until you have fine crumbs with some pea-sized pieces for texture. Mix with melted butter, brown sugar and salt until mixture holds together when squeezed.
Press and bake: Press mixture firmly into bottom and 1 inch up sides of prepared pan. Bake 12 minutes until fragrant and slightly darkened. Cool completely. Reduce oven to 325°F.

RHUBARB:
Roast rhubarb: Toss diced rhubarb with sugar, cornstarch, lemon juice, and vanilla. Spread on a rimmed baking sheet and roast 15-18 minutes until tender but still holding shape.
Drain thoroughly: Transfer to a fine-mesh strainer and let drain 45 minutes, gently pressing to remove excess liquid. Reserve 2 tablespoons of the drained juice for the ginger swirl.

GINGER SWIRL:
Create swirl: In a small saucepan, combine crystallized ginger, cream, honey, butter, and 2 tablespoons reserved rhubarb juice. Simmer 3-4 minutes until slightly thickened. Cool completely.

FILLING:
Beat cream cheese: In a large bowl, beat cream cheese until completely smooth and fluffy, about 3 minutes. Gradually add 3/4 cup sugar, beating until light.
Add egg yolks: Beat in egg yolks one at a time, mixing just until incorporated after each addition.
Combine remaining ingredients: In a small bowl, whisk together custard powder, ground ginger, rhubarb powder, nutmeg, and salt. Add to cream cheese mixture along with sour cream, heavy cream, and vanilla. Beat just until smooth.
Whip egg whites: In a separate clean bowl with clean beaters, whip egg whites until foamy. Gradually add remaining 1/4 cup sugar and whip to soft peaks (peaks should hold but still look slightly wet).
Fold in whites: Gently fold 1/3 of the whipped whites into the cream cheese mixture to lighten it, then fold in remaining whites in two additions. Be gentle to maintain airiness - fold just until no white streaks remain.

ASSEMBLE AND BAKE:
Layer ingredients: Scatter drained rhubarb evenly over crust. Pour filling over rhubarb, spreading gently with a spatula to avoid disturbing the fruit or deflating the airy mixture.
Create swirl: Drop spoonfuls of the ginger swirl mixture over the surface. Use a knife to create elegant swirl patterns, drawing through the mixture in gentle figure-8 motions.
Water bath: Place springform pan in a large roasting pan. Pour hot water halfway up the sides of the springform pan.
Bake: Bake 65-75 minutes until center is almost set but still slightly jiggly when gently shaken. The top should be lightly golden and may puff slightly during baking (this is normal with the whipped whites).
Cool gradually: Turn off oven and crack door open. Cool in oven 1 hour, then remove from water bath and cool completely at room temperature. The cheesecake may settle slightly as it cools - this is expected.
Chill: Refrigerate at least 6 hours or overnight before serving.

TO SERVE:
Run a thin knife around edges before releasing springform sides. Use a sharp knife dipped in warm water and wiped clean between cuts. Garnish with candied rhubarb strips and mint if desired.