Preheat the oven to 180C / 350F / 160C fan forced. Line an 8 inch square baking tin with baking paper.
Sift together the flours and set aside.
Beat together the butter and sugar until light and creamy, scraping down the sides of the bowl as necessary.
Gradually add the flours while continuing to beat on low until it comes together.
Use your hands to press the shortbread mixture into an even layer over the base of the baking tin.
Bake for 20 minutes.
Allow it to cool for 10 minutes, then spread the lemon curd over in an even layer
For the crumble topping
Mix together the flour, almond meal and sugar, then add the butter. Use a pastry cutter, a knife or just you fingertips to gently work the butter into the dry ingredients. If you’re using your fingers, be careful not to melt the butter. It should look like very lumpy wet sand at the end.
Add the egg yolk and lemon zest and use a fork or the same method for the last step to mix it all through the dough. Now you should have a wet dough but make sure to still leave it clumpy rather than turning it into one big smooth ball.
Break the dough up and scatter all over the top of the lemon curd.
Return the tin to the oven and bake for 30-35 minutes until the top is golden
For best results you should always weigh ingredients like flour and sugar. Kitchen scales like these are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).