Heat oven to 350°F (175°C). Line a 9 x 13-inch pan with parchment and lightly oil the paper.
Whisk flour, thandai powder, baking powder, baking soda, and salt in a large bowl.
Whisk yogurt and sugar 3 to 4 minutes until mostly dissolved. Whisk in oil, milk, and vanilla.
Fold in dry ingredients in two additions until just combined. Do not overmix.
Spread batter in the pan. Bake 15 to 20 minutes until a toothpick comes out clean.
Cool in the pan 10 minutes, then on a rack until completely cool, about 45 minutes.
Make the thandai soak
Warm milk until steaming. Simmer 4 to 5 minutes on low if you want a thicker soak (optional).
Stir in thandai powder, sugar, saffron, and rose water (if using). Cool to room temperature.
Make the thandai whipped cream
Whip cold cream to soft peaks. Add thandai milk and powdered sugar. Whip to stiff peaks.
Refrigerate until assembly.
Assemble the jars
Cut 2 1/2-inch cake rounds. You need 12 rounds for six two-layer jars.
Layer in each jar: cake round, 1 to 2 teaspoons thandai milk, whipped cream, pistachios; repeat once.
Top with rose petals. Serve within 2 hours or refrigerate up to 8 hours covered.
Notes
Recipe tested with homemade Small-Batch Thandai Powder; store-bought fine-ground masala works.Caster sugar dissolves faster in the wet mix; whisk granulated sugar until fully dissolved if substituting.Bake cake and thandai milk up to 2 days ahead; whip cream and assemble the day you serve.Use only 1 to 2 teaspoons soak per layer so jars do not turn soggy.Cover leftovers tightly and refrigerate up to 24 hours; best texture the day assembled.