Preheat oven to 170°C. Coat an 8×8 baking dish with nonstick spray or cover bottom with parchment paper. Set aside.
In a large bowl, mix the butter, condensed milk, sugar, egg yolks, baking powder, and salt together on medium speed for about 2 minutes or until mixture is smooth. Turn the mixer to low and add in the flour, mixing until well incorporated and the batter is smooth. Do not overmix!
Pour the batter into the prepared pan. Use an angled spatula to even out the batter then sprinkle with crushed nuts if desired.
Bake for 25-30 minutes, or until the edges are a golden brown.