Grease a 7-inch round cake pan (or similar 6-8 cup capacity pan) generously with butter, then line the bottom and sides with parchment paper—this ensures easy removal after cooking. Set the pan aside. Fill your Instant Pot with 1 cup of water and place the steamer rack inside, then preheat the pot on the sauté setting while you prepare the batter. This head start helps the pressure build faster once the pan is sealed.

In a large mixing bowl, cream together 3/4 cup butter and 1 cup sugar until light and fluffy, about 2-3 minutes. Add the 2 eggs, 1 egg yolk, 1/2 cup yogurt, and 3/4 tsp vanilla essence, mixing until fully combined. In a separate bowl, whisk together 1 1/2 cups flour, 1 1/2 tsp baking powder, 1 1/4 tsp cinnamon, 1/2 tsp salt, 3/4 tsp cardamom, and 1/2 tsp nutmeg. Fold the dry mixture into the wet ingredients until just combined—I recommend using a spatula and folding gently rather than overmixing, which keeps the crumb tender and moist.

In a small bowl, combine 1/3 cup melted butter (cooled to room temperature), 1/2 cup brown sugar, 1/2 tsp cardamom, 1 1/2 cups rhubarb sliced into 1/2-inch pieces, and 1/2 tsp lemon zest. Stir gently until the rhubarb is well coated. The cooled butter prevents the eggs from curdling if it comes into contact with them, and the lemon zest brightens the tart rhubarb flavor beautifully.

Pour the rhubarb mixture into the prepared pan and spread it evenly across the bottom—the rhubarb will sink slightly, which is perfect. Gently pour the cake batter from Step 2 over the rhubarb layer and spread it to an even thickness with a spatula. Cover the top of the pan with a paper towel, then place a sheet of aluminum foil over it, creating a tight seal that prevents condensation from dripping onto the cake during pressure cooking.

Cancel the preheating sauté setting on the Instant Pot. Carefully place the covered pan onto the steamer rack inside the pot using a sling (folded aluminum foil or a silicone sling makes this easier). Seal the lid and set the Instant Pot to cook on high pressure for 60 minutes. Once the cycle completes, allow the pressure to release naturally for 10 minutes, then carefully move the pressure valve to release any remaining pressure manually.

Using oven mitts or tongs, carefully remove the hot pan from the Instant Pot and place it on a heat-safe surface. Remove the foil and paper towel covering. Run a thin knife around the edges of the pan to loosen the cake from the sides, then place a serving plate over the top of the pan. With confidence, flip the pan and plate together so the cake inverts onto the plate, revealing the beautiful caramelized rhubarb on top. Let the cake rest for 5 minutes before slicing—this helps it hold together better.