With the rack in the middle position, preheat the oven to 350 °F (180 °C). Line a mid-sized baking pan (8*8 inches) with a strip of parchment paper, letting it hang over both sides. Butter the parchment paper

In a pot, mix the dates, water, lemon juice, and brown sugar, and bring to a boil (around 10 minutes). Decrease the heat to half, and add the baking soda and simmer for 5 minutes, stirring constantly with a wooden spoon, or until the dates have fallen apart. Take the pot off the stovetop and let it cool.

In a big bowl, combine the oats, flour, brown sugar, and baking powder. Add slightly melted butter and stir to combine.

Spread half the crispy oats mixture from Step 3 in the baking dish and press firmly to form an even layer.

Add another even layer of the date mixture from Step 2. Cover with the remaining crispy oat mixture from Step 3 and press lightly to make it look like a massive cookie.

Bake for 55 minutes until the crisp oats are golden brown. Once cooled down, put the baked dish in the freezer for 2 hours Unmold and cut into 16 squares. Keep the remainder (if there are any), in the fridge for it to be nice and cool for up to 4 days.