Preheat Oven and Prep Pan:

Heat oven to 375°F (190°C). Prepare a muffin pan by lining cups with paper liners or greasing them lightly.

Whisk Dry Ingredients:

In a big bowl, combine 2 cups flour, poppy seeds, baking powder, baking soda, and salt with a whisk. Put aside.

Mix Wet Ingredients:

In another bowl, work lemon zest into ¾ cup sugar until it smells fragrant. Pour in melted butter, eggs, vanilla, buttermilk, and ¼ cup lemon juice; whisk until everything is mixed well.

Combine Wet and Dry:

Slowly incorporate the wet mixture into the dry mixture, folding carefully until just combined. Avoid overmixing.

Make Streusel Topping:

If making streusel: In a small bowl, stir together ¼ cup flour and brown sugar. Work in cold butter until the mixture is crumbly.

Fill Muffin Cups:

Divide batter among the prepared muffin cups, filling them to about ¾ full. If desired, sprinkle streusel on top.

Bake Muffins:

Bake for 18-22 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pan for 5 minutes, then move to a wire rack.

Prepare Lemon Glaze:

For glaze: In a small bowl, whisk powdered sugar with 2-3 tablespoons lemon juice until smooth. Optionally, add vanilla.

Glaze Muffins:

After muffins have cooled completely, drizzle glaze over the tops. Allow to set for a few minutes before serving.