Melt <quantity>1½<unit>cups</unit></quantity> of butter in a saucepan over medium-low heat. Simmer until it is browned and reaches <temperature>150<unit>°C</unit></temperature>. Remove from heat and let it sit for about 30m to cool down

Add <quantity>2</quantity> egg yolks and <quantity>2⅓<unit>tbsp</unit></quantity> of dijon mustard to a blender

Add <quantity>2<unit>tsp</unit></quantity> of salt and <quantity>1<unit>tbsp</unit></quantity> of vinegar

Blend on medium-low speed to combine. Add the brown butter in a thin stream while blending until smooth and fluffy. The brown butter mayo is ready

Peel and finely grate <quantity>200<unit>g</unit></quantity> of potatoes over a kitchen towel

Wrap in a towel. Twist and squeeze to remove most of the liquids

Finely mince <quantity>50<unit>g</unit></quantity> of leek

Finely chop <quantity>¼</quantity> bunch of parsley

Transfer the grated potato to a bowl and add the leek

Add <quantity>2</quantity> eggs and <quantity>25<unit>g</unit></quantity> of all-purpose flour

Add salt and ground black pepper to taste. Mix to combine all the ingredients

Add <quantity>30<unit>g</unit></quantity> of dried white mushrooms to a saucepan and cover with <quantity>500<unit>ml</unit></quantity> of cold water

Bring it to a boil over medium-high heat. Add salt to taste and boil to reduce until you have <quantity>50<unit>ml</unit></quantity>

Drain the liquid into another saucepan. Reserve the mushrooms

Bring it to a boil over medium-high heat. Add <quantity>10<unit>g</unit></quantity> of butter and cook for 2m, while whisking, until the sauce thickens slightly and has a silky texture. Remove from the heat

Small dice the mushrooms, add them to the sauce, and combine. The mushroom sauce is ready

Add <quantity>3<unit>tbsp</unit></quantity> of vegetable oil to a pan over medium heat. Spoon in small amounts of the potato mixture, spreading them to flatten. Fry both sides until golden brown

Transfer them to a tray lined with paper towel to drain excess oil

Stack a few pancakes in a bowl and top them with the Mushroom Sauce to taste

Top with the Brown Butter Mayo and parsley to taste. The deruny with mushroom sauce is ready

Enjoy! Try making this delicious treat for your next brunch.