Make the focaccia dough. Combine the flour, water, salt, and 1 tablespoon olive oil in the bowl of a stand mixer and knead until the dough comes into a ball, about 5-6 minutes. Add additional water if needed, adding 1 tablespoon water at a time. The dough should be soft and elastic but not sticky. Cover the dough with plastic wrap and let rest for 2 hours.
Preheat oven to 350 degrees F.
Make the hazelnut praline. Toast the hazelnuts until lightly browned, about 10 minutes. Remove from the oven and rub the hazelnuts with a clean towel to remove the outer skins. Spread the nuts on parchment paper and set them aside.
Place the sugar in a heavy-bottomed pot and heat it over medium heat. Gently melt the sugar until it starts to brown. Lower the heat to low and continue to cook until the sugar is browned and caramelized, about 4-5 minutes. Gently stir the sugar with a rubber spatula throughout the cooking process to ensure even browning. Immediately pour the caramel onto the hazelnuts and let cool completely. Break the caramelized hazelnuts into small pieces and place them in a food processor. Puree the nuts until smooth, about 7-8 minutes.
Whisk together the mascarpone with 1 tablespoon honey until smooth. Set aside.
Increase the oven temperature to 500 degrees F.
Lightly dust a clean work counter with flour. Divide the dough in half. Working with one portion at a time, roll out the dough. Continue to stretch pull the dough until it's paper-thin, about 14 inches in diameter. You should be able to see your fingers through the dough when it's ready. Repeat with the other half.
Brush a large baking sheet with 2 tablespoon olive oil. Place one of the dough pieces on top. Place about 6-7 dollops of the mascarpone cheese on the dough (for a total of ½ cup), spreading the dollops evenly. Spread the mascarpone just a bit to create a small well in the center of each dollop. Place about 1 ½ tablespoon of the hazelnut praline in each of the mascarpone wells.
Top the focaccia with the remaining dough, pressing the edges together to seal. Trim the excess dough and discard. Gently tear small holes on top of where the filling is. Brush the top with melted butter and sprinkle the coarse sugar. Bake the focaccia for 11-13 minutes or until the top is golden brown.
Remove the focaccia from the oven and dust with powdered sugar. Cut into smaller pieces and serve immediately.