Add all purpose flour, salt and sugar to a large mixing bowl and stir together until well combined. Set aside for now.

Grate the cold butter into a bowl. If the butter starts to melt or is very soft once grated, place the bowl into the freezer. This will help to firm the butter up and make the next step much easier. I cannot stress enough how important it is for the butter to be COLD.

Place the cold, grated butter into the bowl with the flour, salt and sugar and use a spatula, pastry cutter or simply your finger tips to coat the butter in the flour. You want the butter to stay cold, so try to touch it as little as you can.

Now, add the sourdough starter or discard and vinegar and bring the mixture into a dry, shaggy dough. It's best just to use your hands to squeeze the pastry dough together. It will be a rough, dry dough.If the dough really isn't coming together at all, add a little iced water until you are able to bring it together.

Tip the rough dough out onto the counter, divide the dough into two pieces and bring each piece into a rough ball. Wrap each ball of pastry dough in parchment paper and squash it down into a disc shape. Wrap each disc in cling wrap and place both discs into the fridge.If you prefer to just refrigerate as one ball, that's fine too. You can just divide the pastry into two pieces when you want to roll it out.

Leave the dough a minimum of one hour to fully hydrate and become cold. You can leave the dough in the fridge for up to 48 hours before you use it.

Before you start, mix the ingredients for the fillings together:Brown Sugar Cinnamon - combine the ingredients together and mix until well combined. It will be a sandy texture when ready.Strawberry - mix the corn starch and strawberry jam until you can't see the corn starch (the jam will be a little cloudy).

Roll out one disc of pastry until it's around 3mm or ⅛" thick. You want to create 12 rectangles from this dough (I've made my rectangles around 8cm x 12cm (3" x 5") - you can make your bigger or smaller if you prefer).

Once you've cut the 12 rectangles out, lay them on a baking sheet lined with parchment paper (it's ok to use a few trays, this is especially good if you are making 2 different fillings).

Egg wash each of the rectangles (you want egg wash on the whole rectangle, not just the edges.

Add your filling, ensuring you keep it in the middle. You'll want around a tablespoon of filling for each pop tart. You don't want to overfill them as the filling will leak out.

Now roll out the other disc of dough and cut another 12 rectangles, exactly the same size as the bases. Lay a rectangle of dough onto each base, gently sealing them together with your fingers.

Now use a fork to crimp the edges and seal the pastry edges. Pop a few holes in the top of each pop tart using your fork to ensure the topping doesn't burst out.

Egg wash the top of each pop tart.

Bake the sourdough pop tarts at 200C (390F) for around 20 minutes or until the pastry is golden brown.

When the pop tarts are done, remove from the oven and allow to cool on the tray for around 10 minutes before transferring to a wire rack.

While the pop tarts are cooling, mix together the ingredients for the frosting. Whether you're making both types of frosting or just one, all you need to do is combine the ingredients listed until the frosting is smooth.

Once the pop tarts have cooled, spread each one with as much frosting as you like. Leave them for around an hour to allow the frosting to set and enjoy!