Preheat the oven to 180ºC and prepare a baking tray (lined with baking paper, or greased).
In a small saucepan (or a steel measuring cup), simmer the lemon juice until reduced to half the volume.
In a large bowl, mix the sugar with the melted coconut oil.
Add the aquafaba, reduced lemon juice, and vanilla, and mix.
Add the flour, salt, baking powder and poppy seeds, and mix until fully incorporated.
Chill the mixture in the fridge for 30min.
Shape spoonfuls of the dough and place them on the baking sheet, spaced a couple centimetres apart (they shouldn't spread too much).
Press each ball down with a fork, and sprinkle with some extra poppyseeds.
Bake for about 10 minutes, until the bottom edges are slightly golden.
Cool on a wire rack, then store in an airtight container.