Cut beef into 1.5-2 cm (approximately 1 inch) cubes. Mix cognac in a bowl with soy sauce and garlic powder and season with pepper. Add beef to the bowl and mix. Cover the bowl and let rest in the refrigerator for at least one hour. In the meantime, for the skewers, rinse bell peppers, cut in half, remove seeds and ribs and cut into 2 cm (approximately 1 inch) cubes. Rinse tomatoes.
Stick beef, tomatoes and bell peppers on skewers in alternating layers. Brush with remaining marinade and refrigerate until ready to cook. Brush oil on a baking sheet or grill pan. Preheat the oven broiler and cook skewers, watching carefully, for a few minutes on each side. Meat may still be pink inside.
Serve if desired with fresh baguette and salad.