Preheat the oven to 350° F. Either line a muffin tin with paper liners or butter the bottoms and sides generously.
In a large bowl, whisk to combine the flour, sugar, poppy seeds, lemon zest, baking powder, baking soda, and salt.
In a medium bowl, combine the melted butter and egg, and whisk vigorously for 60 seconds until aerated and thick.
Whisk in the sour cream, milk, lemon juice and vanilla until combined.
Pour the wet ingredients into the dry, then stir gently with a spatula to combine the wet and dry ingredients, taking care not to overmix. The batter will be thick.
Evenly divide the muffin batter into the wells. The easiest way I find is to use cookie scoops. You’ll get the perfect amount of batter for 1 large scoop in each plus 1 medium scoop.
Bake the muffins for 22-25 minutes, until a toothpick inserted into the center of the muffins comes out clean. Keep in mind the muffins will not take on a lot of color on top, only a little golden brown on the sides.
When the muffins are done, let them cool in the pan for 5 minutes, then remove the muffins and let cool completely on a wire rack.
In the meantime, make the raspberry butter by placing all the ingredients in a bowl and mashing together with a spoon. I like it when the butter is fully combined but there are still small chunks of fresh raspberries.
To serve, spread the muffins with the raspberry butter as you eat. Enjoy!