Position a rack in the center of the oven and heat it to 175°C. Lightly grease a 23 cm springform pan, line the base with parchment, and wrap the outside in two wide layers of heavy-duty foil. Combine the graham crumbs, 25 g sugar, and melted butter until the mixture resembles evenly damp sand. Press it firmly across the base with a flat-bottomed cup. Bake for 10 minutes, then place the pan on a rack while you prepare the filling.
Reduce the oven to 165°C and bring a kettle of water to a boil for the water bath. Fit a stand mixer with the paddle attachment. Beat the softened cream cheese on low speed for 60 to 90 seconds, scraping the bowl and paddle halfway through, until smooth but not fluffy. Whisk the 200 g sugar with 12 g cornstarch and the salt, then add it gradually. Mix on low for about 2 minutes, scraping twice, until no streaks or visible cream cheese lumps remain.
Mix the sour cream and 10 g vanilla into the cream cheese base on low speed just until uniform. Add the eggs one at a time, mixing for about 15 seconds after each and stopping as soon as the yellow streaks disappear. Scrape the bottom and sides with a flexible spatula rather than increasing the mixer speed. Toss the 170 g mini chocolate chips with the remaining 4 g cornstarch, then fold them through the batter until evenly distributed. The finished batter should look smooth, thick, and minimally aerated.
Pour the filling over the warm or room-temperature crust and smooth the surface with an offset spatula. Tap the pan gently on the counter two or three times to release large trapped bubbles without knocking out every bit of structure. Set the foil-wrapped springform pan inside a large roasting pan. Transfer both pans to the center oven rack, then carefully pour boiling water into the roasting pan until it reaches about halfway up the springform pan. Avoid splashing water onto the batter.
Bake at 165°C for 65 to 75 minutes without opening the door during the first hour. The cheesecake is done when the outer 5 cm looks set and slightly puffed, while a roughly 5 cm circle in the center moves as one soft unit when the roasting pan is nudged. It should not ripple like liquid or appear glossy and wet. Turn off the oven, crack the door open about 5 cm with a wooden spoon, and leave the cheesecake inside for 60 minutes.
Lift the springform pan from the water bath and remove the foil so trapped moisture cannot soften the crust. Place the cheesecake on a wire rack and run a thin knife around the inside edge to release any filling attached to the pan. Cool uncovered for 60 minutes, until the pan no longer feels warm. Cover without touching the surface and refrigerate in the pan for at least 6 hours, preferably overnight. This chill is part of the setting process, not merely a serving preference.
Release the springform ring and slide the chilled cheesecake onto a serving plate, keeping the parchment beneath it if removal risks damaging the crust. Whip the cold heavy cream, confectioners' sugar, and remaining 2 g vanilla at medium speed until medium peaks hold a defined curve. Pipe 12 rosettes around the edge and scatter the remaining 30 g mini chips over them. For neat portions, cut with a thin knife dipped in hot water and wiped completely dry between slices. Serve cool rather than frozen.