1. Prepare the vanilla pastry cream first. Heat milk with vanilla in a saucepan. In a bowl whisk egg yolks with sugar and starch. Slowly add hot milk while whisking, then cook over medium heat until thick and smooth. Remove from heat, stir in butter, cover with plastic wrap touching the surface, and chill until cooled.

2. Preheat the oven and line a square baking pan with parchment paper.

3. Make the shortbread dough by creaming butter and sugar until light. Mix in egg and vanilla. In another bowl combine gluten-free flour blend, almond flour, xanthan gum, and salt. Add dry ingredients to the butter mixture and mix until a soft dough forms.

4. Press about two-thirds of the dough evenly into the bottom of the prepared pan to create the crust. Bake until lightly golden and set.

5. Prepare the rhubarb filling by combining chopped rhubarb with sugar, lemon juice, and starch in a bowl.

6. Spread the chilled vanilla pastry cream evenly over the baked crust. Then distribute the rhubarb mixture on top.

7. Crumble the remaining dough over the fruit layer to create the crumble topping.

8. Bake until the crumble topping turns golden and the rhubarb layer bubbles and softens.

9. Allow the bars to cool completely in the pan so the layers set properly. Then slice into squares before serving.