Step 1: Prepare the Streusel Topping: In a medium bowl, mix together granulated sugar, brown sugar, and all-purpose flour until evenly combined. Add melted butter and stir until crumbly. Chill the mixture.

Step 2: Make the Lemon Curd: In a double boiler, whisk egg yolks, sugar, lemon juice, and lemon zest for about 10 minutes until thick. Remove from heat and whisk in butter until melted. Cool and refrigerate.

Step 3: Prepare the Muffin Batter: Beat cold butter until creamy. Gradually add granulated sugar, mixing well. Add eggs one by one, then yogurt, lemon juice, lemon zest, and vanilla extract until smooth.

Step 4: Combine Dry Ingredients: Whisk together flour, baking powder, baking soda, and salt in a separate bowl. Gradually mix into the wet ingredients until just combined.

Step 5: Fill the Muffin Tins: Preheat oven to 350°F. Line a muffin tin or grease. Spoon half the batter into each cup, add lemon curd, then top with remaining batter and streusel.

Step 6: Bake the Muffins: Bake for 25-26 minutes until golden brown and a toothpick comes out clean. Cool in the pan for a few minutes before transferring to a wire rack.