Preheat oven to 425 degrees. Line a baking sheet with nonstick foil or parchment.
Mix the flour, sugar, baking powder, salt, pudding mix and poppy seeds in a large bowl.
With a pastry cutter, cut in butter until crumbly and coarse.
Make a well in the center and add the lemon zest, vanilla, egg and 1/2 cup of heavy cream. Stir with a spoon or heavy duty scraper until dough comes together. If it seems dry, add another tablespoon or two of creamy. You should be able to bring the dough into a ball with your hands.
Divide dough in half and on a lightly floured surface, shape into 2 6x3 inch rectangles. Cut each rectangle into 4 triangles and place triangles the baking sheet. Brush tops with heavy cream.
Bake for 10-13 minutes or until edges and tops are nicely browned and center is cooked through. Let cool while you prepare the icing.
Put the butter in a 2 cup Pyrex measuring cup and heat in the microwave for about 30 seconds or until melted. Add the powdered sugar and stir well. Mixture will still be powder and dry. Add 1 tablespoon of lemon juice and stir well, then continue adding lemon juice 1 teaspoon at a time until the icing is a good consistency for drizzling or piping. If piping, put in a decorating bag or freezer bag, snip off the end (or corner) and pipe over the scones.