First layer - cookies
In a large bowl stir to combine graham cracker crumbs, sugar, and melted butter. Line a baking pan (25 cm x 20 cm or 10-inch x 8-inch) with baking paper. Press the mixture into the bottom (and sides) of the prepared pan. Place in the fridge for 20 minutes.
First clean and prepare your rhubarb. Remove all the leaves because they are inedible. Roughly chop the rhubarb on 5 cm (2-inch) slices. Pour 100 ml (1/2 cup) boiling water into a small cup. Add the hibiscus tea and set aside for 1 - 2 minutes. Add sugar, lemon juice, vanilla seeds, empty vanilla pod and strained hibiscus tea to a saucepan. Place over high heat, bring to a boil, then add your sliced rhubarb. Cover with a lid and remove from the heat. Leave the mixture to cool to room temperature, then transfer to a fridge, until both the rhubarb and rhubarb syrup are completely cold. Preheat your oven to 160 °C / 320 °F.
Add your cream cheese to a bowl of a stand mixer (fitted with the whisk attachment) or to a large bowl. Beat the cream cheese until completely smooth. Add the sour cream, lemon juice, lemon zest, sugar, powdered sugar, eggs, vanilla seeds and a pinch of salt to the cream cheese and beat to get a nice, creamy, smooth cheesecake mixture.
bake and cool
Pour filling on top of the prepared crust, and spread evenly. Lightly tap on the countertop to release air bubbles. Place in the preheated oven and bake for 35 - 40 minutes at 160 °C / 320 °F or until the edges have set and the middle is still slightly jiggly. Don't worry, the cheesecake bars will firm up as they cool. Remove the cheesecake bars from the oven, and leave to cool to room temperature. Then refrigerate for at least 30 minutes (or overnight).
Transfer the cheesecake to a cutting board and slice the slabs into squares. Serve with cold rhubarb slices and rhubarb syrup. Store in a fridge for up to three days.