Heat half the water and the maple syrup to 110 °F (43 °C) in the microwave, then stir in the yeast. Set this aside for about 5 minutes.
In the bowl of an electric mixer fitted with the paddle attachment, place the flour and the salt. Stir the mixture together then add the bubbly yeast mixture to the bowl along with the rest of the water. Stir to combine on low, then switch to the hook attachment and knead the dough for about 7 minutes until the dough is smooth and just slightly tacky, and the bowl is clean with no dried bits at the bottom. Feel free to adjust the humidity along the way with a sprinkling of water or flour if you feel the dough is too dry or too wet.
Transfer the dough to a greased bowl and cover with plastic wrap. Let rise in a warm, draft-free place (like your oven, turned off but with only the light on) until it has doubled.
Place the butter between two sheets of plastic wrap and roll it into a square (~14×14 cm). Store in the fridge til the dough is ready to use.
Mix together the maple sugar and granulated sugar in a bowl and set it aside.
When the dough has doubled, transfer it onto a lightly floured work surface and roll it into a square, brush off any excess flour, and place the butter block in the middle (as shown in the image above). Fold the corners towards the middle to wrap the butter block with the dough, pinching together the ends and sides to seal in the butter.
Folding: With your rolling pin, pound/roll the dough to a large rectangle and sprinkle it with a ~¼ cup maple sugar, pressing it in a little. Fold it into thirds (like you fold a letter). Rotate the dough 90°. Roll it again into a rectangle, sprinkle with maple sugar, and fold it into thirds again. Wrap the dough in plastic wrap and refrigerate for about 30 minutes.
Sprinkle your work surface with a little maple sugar. Retrieve the dough from the fridge, unwrap it and set it down on your work surface. Repeat the folding sequence again. Then wrap the dough with plastic wrap and refrigerate for another 30 minutes.
Preheat your oven to 400°F (200 °C) (for a more caramelized pastry bottom, preheat a large baking sheet in the oven set in the middle rack).
Roll the dough out thinly (~1 cm if you can) and cut it into 8 pieces. Gently transfer the dough pieces to a parchment lined tray or a parchment-lined whoopie pie pan. You can pinch opposite corners together or leave them as is. Sprinkle with more maple sugar before baking, and a little sea salt if using.
Set the baking sheet over the preheated sheet on the middle rack in the oven, and bake for about 25 to 30 minutes until the pastries are a deep golden brown. Rotate the tray at least once throughout the baking.