Stir together the ingredients for the sauce in a small bowl until well combined.
Add the cheese, heavy whipping cream and paprika into a pot over medium low heat. Stir and simmer until the cheese is melted and the sauce is smooth.
Spread some sauce on the bottoms of the buns.
Add a layer of roast beef over the sauce.
Spread some of the cheese sauce on top.
Add more sauce to the other side of the buns and place them over the cheese sauce.
Brush the top the buns with the butter, and sprinkle with the black sesame seeds before serving.
Notes
If you aren't keto, then feel free to use regular sweeteners, ketchup, and buns. If you are keto, you can buy keto buns (pictured) but those do have some sketchy ingredients. You can also make homemade rolls which is what I normally do, but the sandwiches will be smaller. YOU DECIDE! You can check out my favorite homemade keto rolls here: https://trinakrug.com/the-best-keto-buns-on-the-planet/