Preheat oven to 325 degrees. Lightly grease and flour the sides and bottom of a 9-inch springform pan. Set aside.
Melt butter and chocolate in a heavy saucepan over low heat. Remove from heat and cool to lukewarm.
Add sugar to chocolate mixture. Stir in eggs one at a time, mixing well after each addition. Stir in milk and vanilla.
In a separate bowl, combine flour and salt. Stir into batter until blended.
Spoon batter into prepared springform pan, spreading evenly. Bake in preheated oven for 25 to 30 minutes or until center is firm to touch. Cool on rack before adding cheesecake filling.