Sift flour with baking powder and salt. Cream butter thoroughly, gradually add sugar, creaming until light and fluffy. Blend in eggs, one at a time, beating well after each. Combine milk and vanilla. Add alternately with dry ingredients. Blend thoroughly after each addition. Pour two third of batter into 25 cm tube pan (the bundt is fluted, right? that's what we use), greased on the bottom. Blend into remaining batter the chocolate syrup and baking soda. Pour chocolate batter over white batter. Do not mix. Bake at 180ºC or 45 minutes. Place a sheet of aluminum foil on top and bake for 20 or 25 more minutes.