In a small bowl, sprinkle the yeast over the lukewarm water. Stir to combine and let sit until frothy, about 5-10 minutes.
In the bowl of a stand mixer fitted with a dough blade or a large bowl, combine the flour, sugar, and salt. Pour in the yeast water mixture, butter, and lemon/vanilla into a well in the center of the flour. Process until a soft dough forms or use your hands to gently bring the flour into the liquid until dough comes together. Add more water if needed to bring the dough together or a little more flour if the dough is too sticky to handle.
On a lightly floured surface, knead the dough until smooth and elastic, about 3 minutes. Roll the dough flat and fold in half. Repeat 4-5 times. Cover with a cloth and let rise in a warm room temperature place until puffed, about 1 hour.
Preheat oven to 400˚F (200˚C) and grease a 9x13 inch (23x33 cm) deep-rimmed pan or 2 square pans with butter. The edges should be about 3 inches high.
On a lightly floured surface, roll the rested dough dough into a sheet about 1 inch (2.5 cm) thick. Use a round cookie cutter (about 2 1/2 inches, 6 cm) to cut out circles and place them in the prepared pan. They can be touching or close together. Cover with a cloth and let rise about 30 minutes.
In a small bowl, mix together brown sugar and flour. Brush the tops of the dough with water using your fingertips. Cover generously with the brown sugar mixture.
Bake in preheated oven until sugar is set and the little cakes are lightly golden, 15-20 minutes. Let cool in pan 5 minutes before removing to wire rack.
These Tortitas Negras are best the day they are made.