Preheat your oven to 350°F (177°C) and line an 8-inch square baking pan with parchment paper.

In a mixing bowl, combine 7 tablespoons of softened unsalted butter and 3/4 cup of brown sugar. Beat until light and fluffy, about 2 minutes. Add in 1 teaspoon of vanilla extract and mix until incorporated.

Add 1 cup of all-purpose flour, 1 teaspoon of baking powder, 1/4 teaspoon of salt, and 1 cup of old-fashioned oats into the bowl. Mix gently until it resembles crumbly dough.

Reserve a little more than half of the crumb mixture and press the remainder firmly into the bottom of the baking pan. Bake for 12 minutes until it starts to turn golden.

Whisk together one can of sweetened condensed milk, 6 tablespoons of lemon juice, and 1 tablespoon of lemon zest in a separate bowl until smooth and creamy.

Once the crust is out of the oven, pour the lemon filling over it and spread evenly. Sprinkle the reserved crumb mixture over the top.

Return to the oven and bake for an additional 22–25 minutes until the edges look golden and the center is set.

Allow to cool completely on a wire rack for about an hour. Lift the bars from the pan and cut into squares.