Preheat the oven to 160°C. Grease and line the bottom and sides of an 8-inch (20-cm) square baking tin with baking paper, leaving an overhang for easy removal. Set aside.
In the large bowl. whisk together flour and salt. Set aside.
Place sliced butter ( butter is sliced so that it melts faster and evenly) in a saucepan over medium to low heat. Slowly melt the butter until it is foamy on the surface. Keep stirring until light brown specks begin to form and you smell a nutty aroma. Remove from heat and and pour into a small bowl with all the bits. Leave it to cool .
In a large bowl, combine the brown butter, brown sugar and granulated sugar and stir with a wooden spoon or spatula until combined. Mix in the eggs and vanilla extract until fully combined.
Add the flour mixture and mix until just combined. Fold in half of the mini eggs into the batter, Pour the batter into the prepared tin and spreading it evenly with a spatula or the back of spoon. Gently press in the remaining mini eggs into the batter.
Bake for 30-35 minutes or until a toothpick inserted into the centre comes out almost clean or with a few moist crumbs. Transfer to a wire rack and leave to cool completely.
Using a sharp knife, cut into 2 inch square. Store in an airtight container at room temperature for up to 3 days.