Preheat your oven to 350°F (177°C) and line an 8-inch 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 creamy and smooth, about 2-3 minutes. Add in 1 teaspoon of vanilla extract, 1 cup of all-purpose flour, 1 teaspoon of baking powder, 1/4 teaspoon of salt, and 1 cup of old-fashioned oats. Mix until crumbly.

Press slightly more than half of the crumbly mixture into the bottom of the prepared pan to form the crust.

Bake the crust for 12 minutes until the edges appear lightly golden, then cool slightly.

Whisk together 14 ounces of sweetened condensed milk, 6 tablespoons of fresh lemon juice, and 1 tablespoon of lemon zest until smooth.

Pour the creamy lemon filling over the pre-baked crust, spreading it evenly, and sprinkle with the remaining crumb mixture.

Bake for an additional 22-25 minutes until the edges turn golden brown and the center sets.

Allow to cool completely on a wire rack, then lift out and slice into squares.