Combine the dates, water, brown sugar, and lemon juice in a heavy saucepan. Stirring frequently, bring to a boil over medium-high heat. Then reduce to a simmer over medium-low heat, and cook until the water has been absorbed and the mixture has thickened. (This will take around 5 minutes.)
Remove the pan from the heat, and stir in the vanilla. Allow the mixture to cool completely while you make the crust.
To make the crust and assemble:
Preheat the oven to 350°F (175°C). Lightly grease a 9-inch square baking pan. If you like, first line the pan with parchment paper.
Place the flour, oats, butter, brown sugar, cinnamon, baking powder, and salt in a large mixing bowl. Using an electric mixer on medium speed, mix until combined. (The mixture will be crumbly but should hold together when pinched.)
Mix 1 cup of the crust mixture with the nuts. Set aside to use for the topping.
Press the remaining crust mixture into the bottom of the prepared pan. Next, spread the cooled date mixture on top of the crust. Then, sprinkle the reserved crust mixture over the top of the date mixture, pressing down lightly.
Bake 30 to 35 minutes or until the bars are golden brown.
Cool completely in the pan on a wire rack before cutting into bars.
Notes
Store in an airtight container at room temperature up to 3 or 4 days or in the refrigerator up to a week.