Preheat the oven to 350°f / 180°c. Line a 9” square baking tin with two pieces of parchment paper, forming a “sling” so that you can easily remove the bars.

In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar for 2-3 minutes, until pale and fluffy. Add the eggs, one at a time, mixing to combine before adding the next. Scrape down the bowl. Add the vanilla and mix to incorporate.

In a medium bowl, sift together the flour, baking powder, and salt. Add to the wet ingredients and mix on low until combined.

Weigh out 475g of the mixture, and press evenly into the bottom of the prepared pan, smoothing the surface with the bottom of a drinking glass or with an offset spatula. Set the rest aside.

In a medium bowl, place the rhubarb, tapioca starch, granulated sugar, vanilla bean paste, and salt, and mix with a spatula until well combined. Spread the rhubarb mixture evenly over the surface of the dough - do not press down too hard.

Crumble the remaining dough over the surface of the rhubarb, ensuring that you add it in an even layer.

Bake the bars for 40 to 45 minutes, until the topping is golden brown and the fruit filling is bubbling. If they topping is starting to brown before they are done, lightly cover the pan with foil. Allow to cool completely before using the parchment paper to remove from the pan. Slice into bars, and dust with powdered sugar.

Store leftovers lightly covered with a paper towel at room temperature for up to 3 days. Do not store in an airtight container as they will go soggy.