Make the date filling: Add the dates and water to a stockpot over medium-high heat. Bring to a boil, then decrease the heat to medium-low. Allow the mixture to simmer, stirring occasionally, until the dates soften/break apart and the mixture has thickened (about 10 minutes). Remove from heat and stir in the vanilla. Set aside to cool.
Make the crust: Preheat the oven to 350°F (180°C) and line a 9"x13" (23x33 cm) pan with parchment paper.
Add the butter and brown sugar to a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment. Beat with a hand mixer or the stand mixer until well creamed. Add the flour, oats, baking soda, salt, and cinnamon (if using) and beat until a crumbly mixture forms.
Place half of the dough into the prepared pan. Press into the pan in an even layer. Bake for 7 minutes.
Remove from the oven and spread the date mixture evenly overtop. Top with the remaining dough, evenly sprinkling overtop. Gently press the dough into the date mixture. Bake for another 35 minutes, or until golden-brown.
Allow to cool completely, then slice into 24 bars.