Butter a 6-inch (15 cm) springform pan (see note #1).
In a small pot, bring the water and dates to a boil. Add the baking soda and mix well. Remove from the heat and let cool at room temperature before adding it to the batter.
In a bowl, combine the flour and baking powder.
In another bowl, cream the butter with the brown sugar using an electric mixer. Add the egg and mix until smooth. With the machine running on low speed, add the dry ingredients, alternating with the milk, then add the date mixture, mixing until combined. Pour the batter into the prepared pan.
Prepare a water bath in the pressure cooker by pouring 1½ cups (375 ml) of water into the container and adding the silicone grill. Place the springform pan on the grill (see note #2). Secure the lid and set to High pressure. Set the cooking time to 25 minutes.