Chicken Kiev

1. FOR THE HERB BUTTER: Mix all ingredients in medium bowl with rubber spatula until thoroughly combined. Form into 3-inch square on sheet of plastic wrap; wrap tightly and refrigerate until firm, about 1 hour.
2. FOR THE CHICKEN: Adjust oven rack to the lower-middle position and heat oven to 300 degrees. Pulse bread in food processor to coarse crumbs, about 10 pulses; transfer to large bowl. Transfer crumbs to large bowl, add oil, 1⁄8 teaspoon salt, and 1⁄8 teaspoon pepper and toss until crumbs are evenly coated. Spread crumbs on rimmed baking sheet and bake until golden brown and dry, about 25 minutes, stirring twice during baking. Let cool to room temperature.
3. Slice each chicken breast horizontally, stopping 1⁄2 inch from edges so halves remain attached. Pound between 2 sheets of plastic wrap to even 1⁄4-inch thickness with meat pounder. Pound outer perimeter to 1⁄8 inch. Unwrap herb butter and cut it into 4 rectangular pieces. Pat chicken dry with paper towels, place on counter, and season with salt and pepper. Position breasts cut side up and place 1 piece of butter in center of bottom half of each breast. Roll bottom edge of chicken over butter, then fold in sides and continue rolling to form a neat, tight bundle, pressing on seam to seal. Repeat with remaining butter and chicken. Refrigerate chicken, uncovered, to allow edges to seal, about 1 hour.
4. Adjust oven rack to middle position and heat oven to 350 degrees. Set wire rack in rimmed baking sheet. Combine flour, 1⁄4 teaspoon salt, and 1⁄8 teaspoon pepper in shallow dish or pie plate. Lightly beat eggs and mustard together in second dish. Place bread crumbs in third dish. Working with 1 chicken bundle at a time, dredge in flour, shaking off the excess, then coat with egg mixture, allowing excess to drip off. Coat all sides of chicken bundle with bread crumbs, pressing gently so that crumbs adhere. Place on prepared wire rack.
5. Bake chicken until center of bundles register 160 degrees, 40 to 45 minutes. Let rest for 5 minutes before serving.
   TO MAKE AHEAD: Unbaked, breaded chicken Kiev can be refrigerated overnight and baked the next day or frozen for up to 1 month. To cook frozen chicken Kiev, increase the baking time to 50 to 55 minutes (do not thaw the chicken).

ASSEMBLING CHICKEN KIEV
1. Place butter piece near tapered end of cutlet and roll up end to cover completely. Fold in sides and continue rolling to form cylinder, pressing on seam to seal. Refrigerate.
2. After dredging chilled, rolled chicken bundles in flour, shake off excess, then roll in egg mixture, letting excess drip off.
3. Roll chicken bundles in bread crumbs, pressing to adhere crumbs to chicken.