Place 18 paper baking cups in muffin pans and set aside.
In a mixing bowl, combine cake flour, Granular sugar substitute (Splenda), butter, and mix on medium speed 1 to 2 minutes until blended and crumbly.
Add milk, baking powder, baking soda, and salt to the combined mixture and mix on low speed again.
In another small bowl, combine and blend buttermilk, lemon juice, lemon zest, eggs and vanilla extract. Add poppyseeds to the mixture and combine it with the above flour mixture and mix on medium-high speed until blend and becomes a nice batter. Do not overmix.
Pour muffin batter into prepared pans. Bake muffins 12 to 15 minutes or until toothpick inserted in center comes out clean. Makes 18 muffins.