Make the crumb topping: In a medium bowl melt the butter. Add the flour, brown sugar, and granulated sugar to the melted butter. Using a fork, stir until the mixture is combined and only small pieces remain. Set aside.
Preheat oven to 350°F. Line a muffin tin with cupcake liners. Set aside.
In bowl of a stand mixer fitted with paddle attachment cream the butter and sugar on medium speed for 2 minutes. Scrape down the sides of the bowl. Mix in the eggs, one at a time until combined. Add the vanilla, oil, lemon zest, lemon juice and beat until combined.
Add the salt, baking powder, baking soda, flour, and poppy seeds, and mix until combined (but don’t overmix). Add the sour cream to the batter and mix until no streaks remain
Using a large cookie scoop, portion the batter in the muffin liners. This will make about 15 muffins.
Sprinkle the crumb topping over each muffin.
Bake for 19-20 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Remove from oven and let cool in the pan for 5 minutes. Transfer to a wire rack to cool completely.
Drizzle the lemon glaze overtop each muffin.
To make the glaze: whisk the powdered sugar and lemon juice together in a small bowl.
You can make plain lemon poppy seed muffins by eliminating the crumb topping. The baking times will be similar, but one piece of advice would be to check for doneness about 2 minutes earlier than noted in the recipe.