Adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and line a 9x5-inch (23x13cm) loaf pan with parchment/baking paper.

Add the sugar and lemon zest to a large bowl (or the bowl of a stand mixer, if using), and use your fingertips to rub the zest into the sugar.

Add the softened butter, oil and vanilla, and using a stand mixer fitted with the paddle attachment or a hand mixer fitted with the double beaters, cream everything together until pale and fluffy.

Add the eggs, one at a time, mixing well after each addition, until well combined.Tip: It's very important that your eggs are at room temperature and that you add them one at a time. This way, you'll 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 water for 5-10 minutes before using them.

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

In a jug, mix together together the milk and lemon juice.

Beginning and ending with the dry ingredients, alternately add the dry ingredients (in three batches) and the milk mixture (in two batches) to the butter-sugar mixture, whisking well after each addition, until you get a smooth cake batter with no flour clumps.Tip: This alternating way of adding dry and wet ingredients helps to maintain the emulsion of the butter in the cake batter. When alternating dry and wet ingredients, make sure to always end with the dry.

Transfer slightly less than half of the lemon batter (about 450g) to a separate bowl, and add the ground poppy seeds and the extra milk to this smaller half. Mix well until evenly combined and smooth, the final batter should have a similar consistency to the lemon cake batter.

Using either a spoon or an ice cream/cookie scoop, spoon large dollops of the lemon and the poppy seed batter into the lined loaf pan, alternating the two to create the marbled effect, until no batter remains. (See blog post for photos.)

Use a knife, skewer or small offset spatula to swirl the batters around, making sure that you reach all the way to the bottom of the pan. This will help to create that gorgeous swirl pattern in the final, baked cake. Lightly tap the loaf pan on the work surface to level out the top.

Optional but recommended: Cut cold butter (straight from the fridge) into very thin strips (about 1-2mm or less than ⅛ inch thin) and arrange them in a line on top of the batter, along the middle (see blog post for photos).This creates a natural weak spot on top of the cake and thus controls where the cake will crack during baking – and it guarantees a perfect split along the centre every single time.

Bake at 350ºF (180ºC) for about 1 hour to 1 hour 10 minutes or until well risen, golden brown on top and an inserted toothpick or cake tester comes out clean. The loaf should have a large crack running along the centre.If the cake starts browning too much, cover it with aluminium foil (shiny side up) and continue baking until done.

Allow the cake to cool in the loaf pan for about 10-15 minutes, then transfer it out of the pan and onto a wire rack to cool completely.

Stir the powdered/icing sugar and 5 teaspoons of lemon juice together until you get a thick mixture with no clumps. If your icing is too thick, add an extra 1-2 teaspoons of juice. This should give you the optimal icing consistency. Add the vanilla and mix well to combine.Tip: You can add more lemon juice to make it runnier, or more powdered/icing sugar to make it thicker, as required. However, note that you should make the icing thicker than you might think is necessary, especially if you want to have a generous layer of icing on top of the cake, along with lovely thick drips running down the sides.

Drizzle the lemon icing over the cooled cake, spreading it evenly across the top with the back of a spoon, and letting it drip down the sides. To decorate, you can grate some extra lemon zest on top and sprinkle with poppy seeds, if you wish.

Allow the icing to set and dry out slightly (about 20-30 minutes at room temperature) before slicing and serving.

The gluten free lemon poppy seed cake keeps well in a closed container in a cool dry place for about 3-4 days.