Preheat oven to 350 degrees. Spray a 6 or 12 cup muffin tin or muffin tin liners with non-stick cooking spray. I suggest using liners if you are making jumbo muffins.
In a large bowl, mix together oil, butter, and sugar for 2 minutes until smooth and creamy. Add in eggs, buttermilk, sour cream, lemon juice, and lemon zest. Stir until completely combined.
Fold in flour, baking powder, salt, and poppyseeds. Scoop into muffin tin liners, filling almost to the top (over 3/4 full). Sprinkle with sparkling sugar.
Bake for 18-21 minutes, if using a 12-cup muffin tin. Bake for 23-26 minutes if using a 6-cup muffin tin. Oven temperatures vary so check the center with a toothpick to determine doneness.
Lemon Glaze:
In a small bowl, mix together powdered sugar and lemon juice. Start with 3 Tablespoons of lemon juice and add more for a thinner glaze and more pronounced lemon flavor.
Once the muffins are done baking, let cool for about 10-15 minutes. Place glaze in a piping bag or place in a sandwich-size Ziploc bag and snip one edge of the bag. Run the glaze along the muffin in a zig-zag pattern.
Notes
I suggest using buttermilk but you can use whole or 2% milk instead.