Adjust the oven rack to the lower middle position, pre-heat the oven to 375ºF (190ºC) and line a 9-inch (23cm) square baking pan with parchment/baking paper.Tip: When lining the baking pan, I recommend leaving some parchment/baking paper overhang, as that will help with removing the crumble bars from the pan later on.

In a large bowl, combine the softened butter, sugar and vanilla, and mix well with a wooden spoon or rubber spatula until combined. Tip: Don’t cream or aerate the mixture, as that can make the shortbread too delicate and crumbly (note the absence of any raising agents for the same reason).

In a separate bowl, whisk together the gluten free flour blend, almond flour, xanthan gum and salt.

Add the dry ingredients to the butter-sugar mixture and mix well until it starts clumping together. Then, give it a knead until you get a homogeneous cookie dough.Tip: Because it's gluten free, you don't need to worry about over-working the shortbread dough!

Press about ⅔ of the shortbread cookie dough (about 380g) into the lined baking pan. Smooth it out into an even layer, this is easiest done with a small offset spatula. Set aside until needed.

Set the remaining ⅓ of the cookie dough aside until needed (it can stand at room temperature while you prepare the filling, no need to refrigerate it).

In a bowl, mix together the softened butter, sugar, vanilla and almond extract (if using) with a wooden spoon or a rubber spatula until well combined and smooth. Don't cream or whisk the mixture, you don't want to incorporate too much air into it.

Add the condensed milk and mix well until combined.

Add the almond flour and mix well until you get a smooth, fairly thick batter.

Add the milk and mix well until evenly combined – the final frangipane should have a nicely spreadable consistency.

Dollop the frangipane filling over the bottom shortbread base and smooth it out into an even layer, all the way to the edges.

Crumble the remaining ⅓ of the shortbread cookie dough over the frangipane filling, making sure that you get a fairly even coverage with some of the filling peeking through.

Sprinkle evenly with the flaked almonds.

Bake at 375ºF (190ºC) for about 30-35 minutes or until the crumble and the flaked almonds are deep golden brown.

Allow the bars to cool completely to room temperature before you remove them from the baking pan (with the help of the overhanging parchment/baking paper). Dust generously with powdered/icing sugar, slice and serve.Tip: I really recommend cooling the almond bars

The gluten free almond crumble bars keep well in a closed container in a cool, dry place for about 3-4 days.