Prepare cake batter according to package directions, adding lemon extract. Pour into a greased and floured 9-in. round baking pan. Bake at 350° for 25 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
In a bowl, whisk milk and pudding mix for 2 minutes; refrigerate. In a heavy saucepan or microwave; melt chocolate and butter, stir until smooth. Stir in confectioners' sugar until crumbly. Stir in hot water, 1 teaspoon at a time, until smooth. Split cake into two horizontal layers. Spread pudding on bottom layer. Place second layer over pudding. Pour chocolate glaze over top, letting it drizzle down the sides. Store in the refrigerator.