• Preheat oven to 350°F (175°C). Line an 8x8 or 9x9-inch pan with parchment paper, leaving overhang on the sides for easy removal.

Making the crumb topping

• In a medium bowl, whisk together flour, brown sugar, granulated sugar, cinnamon (if using), and salt. Stir in the melted butter until clumps form. Set aside.

Make the batter

• In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
• In a separate large bowl, cream the butter and sugar together until light and fluffy. Beat in the eggs one at a time. Mix in the Greek yogurt, lemon zest, lemon juice, and vanilla extract until combined.
• Gradually add the dry ingredients to the wet, mixing just until combined (do not overmix).

Assemble

• Spread the batter evenly into the prepared pan. Sprinkle the crumb topping evenly over the top.
• Bake for 35–40 minutes or until a toothpick inserted in the center comes out clean and the top is golden brown.
• Cool completely in the pan. Once cooled, lift out using the parchment paper and slice into bars.

Glaze

• In a small bowl, whisk together powdered sugar and lemon juice until smooth. Drizzle over the cooled bars.