Sift the cake flour ans baking powder into a medium bowl. Add the salt and whisk to combine.
Combine the sugar, eggs and vanilla paste in a deep medium bowl and mix with an immersion blender. Add the dry ingredients in 2 additions, mixing until just combined. With the blender running, pour in the melted butter in a steady stream and continue to mix until the batter is smooth. Add the lemon juice and blend again to combine.
Fold in the lemon zest and poppy seeds. Transfer the batter to a covered container and refrigerate overnight, or for up to 36 hours.
Preheat oven to 425 degrees.
Line the muffin tin with muffin papers and spray the papers with nonstick spray.
Transfer the batter equally into each cup, stopping approx 3/8 inch from the top. (I made 12 muffins, and the batter fills up all 12 equally).
Place the muffin tin in the oven, lower the temperature to 325 degrees, and (for jumbo muffins) bake for 34-37 minutes, or (for regular sized muffins) bake 20-25 minutes or until the muffins are golden brown and a skewer inserted in the center comes out clean. Set the pan on a cooling rack and cool completely.
The muffins are the best the day they are made, but they can be wrapped individually in plastic wrap and stored in a covered container at room temperature for up to 3 days or frozen for up to 1 week.