Preheat oven to 350°F. Line on bottom and sides an 8 x 8 or similar size pan with parchment paper.
In a saucepan, combine the dates, sugar and water. Cook over medium-high heat, stirring frequently until very thick. Remove from heat and stir in walnuts, vanilla and orange liqueur. Allow to cool.
To a large bowl or food processor add the flour, oats, brown sugar, baking powder, orange peel and butter. Mix or pulse until combined and dough just begins to stick together. Transfer 1/2 of the dough to the baking pan and press down into the pan with the bottom of a measuring cup. Bake in preheated oven for 10 minutes. Cool slightly then pour date filling over baked crust. Sprinkle remaining crust dough over the top.
Bake in oven for 35 - 40 minutes until browned and bubbling on the sides. Remove from oven and let cool in pan. Remove from the pan using the sides of the parchment paper, cut and serve. For cleaner cutting, chill for at least 1 hour prior to cutting.