Mash the tomatoes with a whisk in a small nonreactive saucepan over high heat for about 1 minute. Add the garlic, Shrimp Stock, salt, cayenne, and black pepper and bring to a boil, whisking. Continue cooking, whisking constantly, for about 1 minute.
Add the butter a few pats at a time, and when half incorporated, remove the pot from the heat and continue whisking until all of the butter is incorporated.
Serve immediately or prepare only through Step 1; store, refrigerated, in airtight container. Reheat in a saucepan over low heat. When the sauce comes to a simmer, proceed with Step 2.
Yield: 1 1/2 cups