With the rack in the middle position, preheat the oven to 350 °F (180 °C). Line a 20-cm (8-inch) square baking dish with a strip of parchment paper, letting it hang over both sides. Butter the other two sides.
In a pot, bring the dates, water, lemon juice, and brown sugar to a boil. Add the baking soda and simmer for 5 minutes, stirring constantly with a wooden spoon, or until the dates have fallen apart. Let cool.
In a bowl, combine the oats, flour, brown sugar, and baking powder. Add the butter and stir to combine.
Spread half the crisp in the baking dish and press firmly. Layer with the date mixture. Cover with the remaining crisp and press lightly. Bake for 55 minutes or until the crisp is golden brown. Let cool on a wire rack for 4 hours. Unmould and cut into 16 squares.