Spray a 9 x 13-inch pan with cooking spray and line the bottom with parchment paper. Set aside. Preheat the oven to 350°F.
In the bowl of a stand mixer fitted with the paddle attachment, add the butter and granulated sugar. Turn the mixer to medium-high and mix until light and creamy, about 2 minutes.
Stop the mixer to scrape down the bowl with a spatula. Add the flour, salt, nutmeg and cardamom. Turn the mixer to low and mix until thoroughly combined. Place the dough into the greased baking pan and press firmly to evenly coat the bottom of the pan, creating a 1/2-inch base. Prick the surface of the shortbread with a fork. Place the pan in the freezer for 10 minutes.
Place the pan in the oven and bake for 15-20 minutes, or until the crust is a very light brown. Remove from the oven and set aside to cool while you prepare the filling.
Citrus Filling
In a medium bowl add the sugar, flour, lemon zest, blood orange zest and salt. Whisk to combine. Add the eggs, fresh squeezed citrus juices and vanilla. Whisk to combine. Pour the filling over the par-baked shortbread crust, making sure it spreads evenly.
Place the pan in the oven and bake for 25-30 minutes, or until the filling sets and the edges are slightly brown--be careful not to over-bake the bars or they will be rubbery!
Remove the pan from the oven and let cool on a baking rack, placing the bars in the fridge once they’re room temperature. Let set in the fridge for 2 hours before cutting the bars into 3 x 2-inch rectangles. Dust with powdered sugar and serve.