Adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and prepare 9”x5” loaf pan (2lb/900g) with parchment.
In a bowl, sift together the gluten free 1:1 flour blend (w/xanthan), almond flour, baking powder, and salt. Set aside.
Add the sugar and orange zest to a bowl and use your fingertips to rub the zest into the sugar. This helps to release more essential oils from the zest.
Cream the butter and sugar (with the added orange zest) together until pale and fluffy. This can be done with a hand mixer or in the bowl of a stand mixer fitted with a paddle attachment.
Add the eggs, one at a time, mixing well after each addition, until well combined. It's very important that your eggs are at room temperature and that you add them one at a time to maintain the emulsion of the butter as much as possible. If you forgot to take your eggs out of the fridge, place them in warm tap water for 5-10 minutes before using them.
Add the vanilla and mix until combined.
Beginning and ending with the dry ingredients, alternately add the dry ingredients (in three batches) and the orange juice (in two batches) to the butter-sugar mixture, whisking well after each addition, until you get a fairly smooth cake batter with no flour clumps.
Transfer the cake batter into the prepared loaf tin, smooth out the top, and bake in the pre-heated oven at 350ºF (180ºC) for about 1 hour - 1 hour 5 minutes or until risen, golden brown on top and an inserted toothpick comes out clean. The loaf should have a large crack running along the center.
Allow to cool in the loaf pan for about 10-15 minutes, then transfer the cake out of the tin and onto a wire cooling to cool completely.