Prepare the Streusel: In a mixing bowl, combine granulated sugar, light brown sugar, and all-purpose flour. Pour in melted butter, using a fork to mix until the mixture resembles coarse crumbs. Cover the bowl and chill the streusel mixture in the refrigerator.

Make the Muffin Batter: Cream together softened unsalted butter and granulated sugar until light and fluffy, about 3-4 minutes. Add the eggs one at a time, mixing well after each addition. Stir in the lemon zest, Greek yogurt, lemon juice, and vanilla extract until smooth and creamy.

Combine Ingredients: In a separate bowl, whisk together all-purpose flour and baking powder. Gradually add the dry mixture to the wet batter, folding gently until just combined. The batter should remain slightly lumpy.

Fill Muffin Tins: Preheat the oven to 350°F (175°C). Line a muffin pan with paper liners and scoop the muffin batter into each cup, filling about two-thirds full. Create a small well in the center of each muffin to hold the lemon curd. Spoon about one teaspoon of lemon curd into each well.

Top with Streusel: Sprinkle an even layer of the chilled streusel over each muffin, covering the lemon curd.

Bake: Place the muffin pan in the oven and bake for 25-26 minutes, or until the tops turn golden brown. Use a toothpick to check for doneness.

Cool: Let the muffins cool in the pan for about 5 minutes, then transfer to a wire rack to cool completely.