Preheat oven to 340F. Warm milk to body temperature and butter to room temperature. Shift dry ingredients together.
Beat butter until fluffy then add sugar and beat more. Add the beaten egg slowly, then add warmed milk slowly until well combined.
Fold dry ingredients into the mixture.
Pour the batter into the muffin pan. Insert chunked chocolate into the batter.
Bake for about 25 minutes.