Valentine's Ombré Pancake Cake

Ingredients

• 6 tbsp unsalted butter (plus more for cooking)
• 500g plain flour
• 2 tbsp sugar
• 1 tsp salt
• 6 large eggs
• 1.5l whole milk
• 4 large egg whites
• 200g granulated sugar
• 1/4 tsp salt
• 454g unsalted butter, room temp
• 1 tsp cream of tartar
• 79ml water