Place skillet in cold oven and turn oven on to preheat to 450° F. (with skillet inside, to pre-heat, as well)
Poach eggs: In a large, deep skillet, heat several inches of water to a boil over high heat, then reduce heat to simmer (medium-low). Crack eggs into small dishes/ramekins and gently slide in to simmering water. Stir gently if necessary to gather cooking egg white around yolk. Allow to cook until just set, about 3 minutes (yolk should be very runny, but white should be set). Immediately transfer to a bowl of cold water. Set aside.
Make Dutch baby batter: Combine all the batter ingredients in a blender and blend for 30 seconds, or until well mixed and frothy.
When oven is pre-heated, carefully remove hot skillet from oven and add the 1 Tbsp. of butter to the pan. Swirl pan quickly to coat the bottom, but don't worry about it completely melting. If Dutch baby batter has sat for a few minutes, give it a quick blend, then add batter to the hot pan. Immediately place pan back in the oven and set timer for 13 minutes.
Make the Hollandaise: While the Dutch baby is cooking, make the hollandaise sauce. Rinse out your blender and dry. Melt the 1/2 cup of butter in a small saucepan until melted and foamy. Add the 3 eggs yolks to the clean blender and blend for 30 seconds, or until well mixed. Add the salt, pepper and lemon juice (I start with 1 Tbsp. and then usually add a bit more at the end after tasting) and blend to combine. When the butter is ready, with the blender running, begin adding the butter to the egg yolks VERY SLOWLY (a drop at a time at first, increasing to a slow, steady stream). I do this through top blender top by removing the clear cap on the lid. If for some reason you don't have that, simply remove the lid and cover most of the blender top with a tea towel instead, to protect against splatters. By the time all of the butter has been added, the sauce should be thickened and lightened. Taste and adjust seasoning to your taste (more salt/pepper or lemon juice, to taste).
Check Dutch baby at 13 minutes of baking. It should be puffy and very deep golden around the edges. It may need to bake a few minutes longer. Remove from oven when done and allow to stand. Drain cold water from poached eggs and replace with warm water. Let stand in the warm water for a minute, then transfer the eggs to a paper towel to drain, then to the top of the warm Dutch baby. Add scrunched up ham slices around the eggs, then top it all with the hollandaise sauce. Season with some salt and pepper and garnish with a sprig of fresh parsley and enjoy!