Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned and crisp, about 10 minutes. Drain on paper towels; set aside.

Stir milk and unsweetened chocolate in a large saucepan over low heat until chocolate melts and mixture is thoroughly combined, 2 to 3 minutes; stir in sugar all at once. Whisk mixture, trying to clean the sides of pan as much as possible.

Bring the mixture to a boil and cook, stirring constantly, until a candy thermometer inserted into the middle of the fudge reads 236 to 238 degrees F (110 to 115 degrees C), or candy forms a soft ball when dripped into cold water. Remove from heat as soon as candy hits the correct temperature; stir in bacon.

Place butter squares onto fudge mixture and pour vanilla extract over the top. Set aside with thermometer in the fudge; do not disturb pot until temperature has dropped to 110 to 115 degrees F (43 to 46 degrees C).

While fudge is cooling, line an 8x8-inch baking dish with aluminum foil and coat with butter. When fudge has reached 110 to 115 degrees F, stir in butter and vanilla extract thoroughly into the fudge; pour fudge into prepared pan.