An Heirloom Tomato BLT sandwich is the ultimate way to celebrate summer tomatoes, and don't even think about skipping the brown butter mayonnaise, it's mandatory.
In a small skillet or saucepan over medium heat, melt the butter completely; cook until it turns brown and smells nutty, about 4 to 6 minutes. Be sure to stir frequently, scraping up any bits from the bottom so they don't burn. Take the pan off the heat and transfer the brown butter to a glass measuring cup with a spout. Let cool to just above room temperature. Combine egg yolk, lemon juice, cold water, and salt in a medium mixing bowl. Whisk until well integrated and bright yellow, about 30 seconds. Whisking constantly, add about half of the brown butter to the yolk mixture, a few drops at a time. Do not add the brown butter too rapidly; or an emulsion will not form.
Gradually add remaining brown butter in a very slow, thin stream, whisking constantly, until mayonnaise is thick and spreadable. The process should take around 6-8 minutes (this step can also be done in a food processor. Season to taste.
Layer bacon, tomato and lettuce over a slice of bread and top with the other slice.
The mayo also makes for an amazing addition to any salad dressing