In a large bowl, whisk together the flour, baking powder, salt, and sugar until well combined.

In another bowl, mix the milk, vegetable oil, and egg until smooth.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are fine.

In a medium bowl, combine the shredded brisket, maple syrup, and Dijon mustard. Mix well until the brisket is evenly coated.

Preheat your waffle iron according to the manufacturer's instructions.

Lightly grease the waffle iron with cooking spray or butter. Pour enough batter into the waffle iron to cover the surface. Close the lid and cook until golden brown and crispy, about 5 minutes. Repeat with the remaining batter.

Once the waffles are cooked, place a slice of cheese on one waffle, followed by a generous portion of the brisket mixture, and top with another slice of cheese. Finish with a second waffle on top.

Heat a skillet over medium heat and add a little butter. Carefully place the assembled waffle sandwich in the skillet. Cook for about 3-4 minutes on each side, or until the cheese is melted and the waffles are golden brown.

Remove the sandwich from the skillet, slice it in half, and serve warm.