Preheat the oven to 180°C/355°F. Grease and line a 9"x9" (23cm x 23cm) square cake tin with a baking paper overhang.
Combine the dates, water, sugar, salt, and lemon juice in a medium saucepan. Cook over medium heat (10-15 minutes) until the date mixture is thick and the dates have absorbed all the water. Set aside and allow to cool.
240 grams dates, 236 mL water, 2 tablespoon white granulated sugar, 2 tablespoon lemon juice, ¼ teaspoon table salt
Combine the butter, icing sugar, white sugar, and vanilla in a large mixing bowl. Beat together until the butter and sugars are well incorporated, light, and fluffy and the butter has lightened in color.
Press ½ to ⅔ of the dough into the bottom of the prepared tin. Use the back of a cup measurer or offset spatula to really press the dough into the pan. Prick the dough with a fork throughout and bake for 15 minutes until the biscuit base is a nice light golden brown.
Pour the date mixture over the shortbread base and smooth out using the back of the spoon, or an offset spatula.
Crumble the leftover biscuit dough over the top of the date mixture. Bake for 20-25 minutes or until the top is golden brown.
Remove from the heat. Allow to cool in the tin for 30 minutes before lifting it out and allowing it to cool on a cooling rack completely.
Once the slice has cooled completely, slice it into 20-25 squares. Serve and enjoy.
All temperatures stated are for conventional ovens. Decrease the temperature by 20°C/25°F or gas mark 1 for "fan" or air-fryers.
Notes
Store this date and oat slice in an airtight container for up to a week. Or freeze for up to 3 months.