For the scones: Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper or silicone mats.
Sift together the flour, granulated sugar and baking powder. Cut the cold butter into pats, then use a pastry cutter or 2 knives to cut the butter into the flour mixture. Keep going until the mixture is crumbly, then stir in the poppy seeds.
Mix together the heavy cream, lemon zest and juice and egg in a small pitcher. Stir the wet ingredients into the flour mixture with a fork or spatula just until the dough comes together.
Turn the dough out onto a floured surface and lightly press it together until it forms a rough rectangle (the mixture will be pretty crumbly). Use a rolling pin to roll into a rectangle about 12 inches by 7 inches and 1/2 to 3/4 inch thick. Use your hands to help with the forming if necessary. Then cut the rectangle into 12 symmetrical rectangles, first cutting lengthwise into 3 strips and then across into 4 strips. Next, cut each rectangle in half diagonally, to form 2 triangles. Transfer to the prepared baking sheets.
Bake until cooked through but not yet golden, about 16 minutes. Allow to cool for 10 minutes on the baking sheets, then transfer to a cooling rack to cool completely.
For the lemon glaze: Whisk together the powdered sugar, milk, lemon zest and juice and salt until completely smooth, adding more milk if needed for thinning.
Dip the scones in the glaze, then sprinkle with poppy seeds. Allow the glaze to set completely before serving.