Preheat oven to 350 degrees Farhenheit.

Mix butter and brown sugar until well combined.

In a separate bowl, sift together flour, salt, baking powder, and oats.

Add oat and flour mixture to butter/sugar mixture and combine well.

Press half of the resulting mixture into the bottom of an 9x13 inch pan. (Or for thicker bars, put it in an 8x8 pan)

In another bowl, whisk together sweetened condensed milk, lemon juice, and lemon zest - spread on top of oat mixture.

Spread the remaining oat mixture over the sweetened condensed milk layer evenly. (Do not pack down, just sprinkle it all on.

Bake for 20-25 minutes, or until golden brown