Toast the sourdough: Add 2 tbsp butter to a skillet over medium heat. Add slices of sourdough bread and cook for 2 to 3 minutes per side, or until golden brown. Repeat with the remaining 2 slices of bread.
Make the smoky mayo: In a small bowl, combine the Kewpie mayonnaise, minced garlic, sweet smoked paprika, and a squeeze of lemon juice. Stir until smooth, then season to taste with salt and black pepper.
Season the tomatoes: Slice the heirloom tomatoes and season with flaky sea salt and black pepper.
Make anchovy butter: Heat 4 tbsp butter in a small pan on low heat. Once the butter has melted, add the anchovies and use the end of a wooden spoon to break them up and dissolve into the butter. Zest the lemon and add, stirring. Remove from the heat. Squeeze in the lemon juice and season with lots of ground black pepper.
Assemble: Add a nice layer of smoky mayo to each toast. Top with 2 slices of tomato and drizzle with anchovy butter. Finish with chopped chives. Enjoy!