Preheat oven to 450℉ and set oven rack to the center position. Place a large baking sheet on rack to catch any oil or dripping that may occur. In a medium size bowl whisk together milk and eggs. Add flour, salt and melted butter and continue to whisk until smooth, approximately 1 minute.
Using a mini muffin tin, add 1 tsp of oil to 20 mini cups. Place on prepared baking sheet set on the middle rack for serval minutes to heat oil. Once hot, remove mini muffin pan and quickly scoop 1 tbsp of batter into each of the 20 cups. Place back in oven and bake for 18 minutes. Remove from oven and tilt each mini yorkshire onto its side.
Stuff approximately 10g of warm roast beef into the center of each mini yorkshire (a slit or hole should generate naturally at the top while baking). Pipe or squeeze approximately 1 tsp of horseradish sauce into the same hole. Serve immediately with au jus if desired.
While yorkshires are baking, whisk together all ingredients in a small bowl. Scoop into a piping bag or squeezable condiment container. Keep refrigerated until ready to use.
I used shaved deli roast beef for this recipe, however left over homemade roast beef, flaked, sliced or shaved is just as good, if not better.