Preheat oven to 350. Sift together flour, sugar, baking powder, and salt. Combine butter, milk, and extract. Add dry ingredients to wet and beat 2 minutes. Add eggs and beat 2 minutes longer. Pour into unbaked pie shell. Melt chocolate in boiling water. Add sugar and bring to a boil. Remove from heat and add butter and vanilla. Pour sauce over pie batter. Sprinkle nuts over the top. Bake 55-60 minutes or until done. Garnish with whipped cream and chocolate shavings.