Heat oven to 350°F.
Grease and flour one 9-inch round baking pan.
Stir together flour, sugar, cocoa and baking soda in large bowl.
Add butter, milk, egg and vanilla.
Beat on low speed of mixer until all ingredients are moistened.
Beat on medium speed 2 minutes.
Pour batter into prepared pan.
Bake 30 to 35 minutes or until wooden pick inserted in center comes out clean.
Cool 10 minutes; remove from pan to wire rack.
Cool completely.
Cut cake into two thin layers.
Place one layer on serving plate; spread filling over layer.
Top with remaining layer.
Pour onto top of cake, allowing some to drizzle down sides.
Refrigerate until serving time.
Cover; refrigerate leftover cake.