In a medium bowl, whisk together baking soda, salt and flour, and reserve.
In a double boiler set over gently simmering water, stir together chocolate, 1 cup sugar, 3/4 cup milk and 1 egg. Keep stirring while the chocolate melts, and continue cooking, stirring, until the mixture becomes thick and glossy, about 5 minutes. Remove from heat and allow mixture to cool to room temperature.
In a bowl of an electric mixer on medium speed, beat together butter and remaining 1 1/2 cups sugar until light and fluffy, about 2 to 3 minutes. Add remaining 4 eggs, one at a time, beating well after each addition. Add vanilla extract and beat until thoroughly combined. Reduce speed to low and add flour mixture in 3 parts, alternating with remaining 1 cup milk and beginning and ending with flour mixture. Add reserved chocolate mixture and mix until combined.
Divide batter evenly between the prepared pans and bake until a sharp knife or toothpick inserted in the center comes up clean, about 25 to 35 minutes. Remove pans from oven and transfer to a wire rack and cool for 15 minutes, then remove cakes from pans and cool thoroughly on a wire rack before frosting with Chocolate Buttercream Frosting.
Enough to frost a 2-layer cake.
From "Savoring the Seasons of the Northern Heartland," by Beth Dooley and Lucia Watson.