Step 1: Prepare the Caramel - Combine the white granulated sugar and water in a saucepan over medium heat. Stir gently until sugar dissolves. Simmer without stirring until golden amber, about 8-10 minutes. Pour into each ramekin.
Step 2: Make the Custard Mixture - Whisk together cream, milk, large eggs, egg yolks, caster sugar, vanilla extract, and brewed espresso in a mixing bowl. Add espresso powder if using.
Step 3: Assemble the Ramekins - Pour the custard mixture over the caramel in the ramekins, filling them about ¾ full.
Step 4: Prepare the Instant Pot - Place the steamer rack in the Instant Pot, add 1 cup of water, and arrange the filled ramekins on the rack.
Step 5: Cook the Custards - Seal the lid, set the valve to sealing, and cook on high pressure for 8 minutes. Allow natural pressure release for 10 minutes, then quick release.
Step 6: Cool and Serve - Remove ramekins, cool at room temperature for 30 minutes, then refrigerate for at least 4 hours. To serve, run a knife around edges, invert onto a plate.
