• Pre-heat oven to 350°
• Butter a 9-inch (23cm) square cake pan.

Topping

• Mix very cold butter, sugar, flour and oats with your hands until all is well mixed with peas size crumbs. Set in Refrigerator until needed

Batter

• Cream butter, sugar and lemon zest together until light and creamy, about 3 minutes. (by hand with a wooden spoon or using an electric mixer)
• Add the eggs one at a time, fully incorporating. Make sure to scrape down the sides of the bowl
• Mix together the flour, baking powder, baking soda and salt in medium-sized bowl.
• Add half the flour mixture to the creamed, mixing just enough to incorporate
• Add Greek yogurt, mixing in just enough to incorporate
• Add the remaining flour mixture, mixing just enough so it’s barely incorporated (there will still be dry bits of unincorporated flour)
• Using a flexible spatula, fold the blueberries into the mixture. Being careful not to overmix.
• Scrape the batter into the prepared baking pan and smooth the top.
• Add the topping over the batter and bake for 50-55 minutes at 350 degrees. Or until the buckle is lightly browned on top and feels just set in the center. It should spring back lightly when you touch the center.
• When the buckle is almost finished baking, make the syrup by heating the lemon juice and sugar in a small saucepan over medium heat, cooking it until it thickens. It should take about 5 minutes. Don't allow the syrup to brown.
• Remove the buckle from the oven and pour the warm lemon syrup over it, letting it soak in.
• Serve buckle slightly warm or at room temperature with vanilla ice cream or home made whipped cream.