Preparation Grind porcini to a powder in an electric coffee/spice grinder or blender. Cook garlic in oil in a 10-inch heavy skillet over moderate heat, stirring, until fragrant, about 1 minute. Add tomato paste and porcini powder and cook, stirring, 2 minutes. Tilting skillet, press paste with back of a spoon to separate from oil, then pour off oil from skillet into a small cup and reserve. Transfer paste to a plate and cool slightly, about 5 minutes. Prepare a gas grill for indirect-heat cooking over moderate heat. Pat chicken dry and remove excess fat. Working from backbone side of a half chicken, loosen skin over breast and drumstick to form a large pocket, then smear half of paste over meat. Repeat with remaining chicken half and paste. Rub reserved oil all over chicken and sprinkle with salt and pepper. Grill chicken, skin side up, over turned-off burner, covered, without turning, 20 minutes. Turn chicken over and grill, covered, until skin is crisp and meat is cooked through, 5 to 10 minutes more. Cooks' note:Halved chicken can be roasted, skin side up, in a foil-lined shallow baking pan in upper third of a 475°F oven 30 minutes, then broiled 4 to 6 inches from heat until skin is crisp.
Preparation Grind porcini to a powder in an electric coffee/spice grinder or blender. Cook garlic in oil in a 10-inch heavy skillet over moderate heat, stirring, until fragrant, about 1 minute. Add tomato paste and porcini powder and cook, stirring, 2 minutes. Tilting skillet, press paste with back of a spoon to separate from oil, then pour off oil from skillet into a small cup and reserve. Transfer paste to a plate and cool slightly, about 5 minutes. Prepare a gas grill for indirect-heat cooking over moderate heat. Pat chicken dry and remove excess fat. Working from backbone side of a half chicken, loosen skin over breast and drumstick to form a large pocket, then smear half of paste over meat. Repeat with remaining chicken half and paste. Rub reserved oil all over chicken and sprinkle with salt and pepper. Grill chicken, skin side up, over turned-off burner, covered, without turning, 20 minutes. Turn chicken over and grill, covered, until skin is crisp and meat is cooked through, 5 to 10 minutes more. Cooks' note:Halved chicken can be roasted, skin side up, in a foil-lined shallow baking pan in upper third of a 475°F oven 30 minutes, then broiled 4 to 6 inches from heat until skin is crisp.