Preheat oven to 350 degrees F (180 degrees C) and lightly butter or line 12 muffin cups with paper liners. In the bowl mix the flour, sugar, baking powder, and salt until combined.
Add the butter, eggs, vanilla extract, lemon extract and sour cream (or yogurt), in another bowl and whisk for few minutes.
Mix wet and dry ingredients together, at medium speed, until the batter is smooth and satiny, about 30 - 60 seconds.
Evenly fill the muffin cups with the batter and bake for about 20 - 25 minutes or until lightly browned and a toothpick inserted into a cupcake comes out clean. Remove from oven and place on a wire rack to cool.
Once the cupcakes have cooled completely, cut about an 1/2 inch (1.5 cm) hole into the center of each cupcake, small spoon. Then spoon (or pipe) a heaping teaspoon of lemon curd into each indentation.
To make Lemon Curd: In a stainless steel bowl, placed over a saucepan of simmering water, whisk together the eggs, sugar, and lemon juice until blended.
Cook, stirring constantly with a heatproof rubber spatula or wooden spoon (to prevent it from curdling), until the mixture becomes thick (like sour cream or a hollandaise sauce) (160 degrees F or 71 degrees C). This will take approximately 10 minutes. Remove from heat and whisk until the butter has melted. Add the lemon zest and let cool. The lemon curd will continue to thicken as it cools. Cover immediately (so a skin doesn't form) and refrigerate until cold (preferably overnight). The lemon curd can be made several days (up to a week) in advance.
To make the butter cream, whisk the butter until smooth and then add the icing sugar gradually.
Combine and whisk well, add the cream, lemon extract. Whisk well until a smooth consistency is attained.
Mix the colour into it and pour into piping bag, pipe the cold cupcakes.