Because of the sticky dulce de leche in the bottom, I use standard 4-ounce (125ml) porcelain ramekins and serve the cakes right in the ramekins. But you can use any kind of ovenproof custard cup for this recipe; the mixture will rise a bit in the oven, but not enough to overflow.
Do be sure to let the cakes cool for at least 5 minutes before serving, and let guests know the little molds are still hot. To cool things down, serve these with a scoop of vanilla or coffee ice cream, or pass a pitcher of cold, heavy cream for guests to help themselves.
Note: You can find dulce de leche at well-stocked supermarkets or stores that specialize in Mexican and South American products, or you can make your own, using the recipe on my website. —David Lebovitz
Butter six ramekins or custard cups. Dust each with cocoa powder and tap out any excess. Put a heaping tablespoon of dulce de leche in each cup, then divide the flaky salt among them by sprinkling it over the dulce de leche. Put the custard cups on a baking sheet.
In a bowl set over a pan of simmering water, melt the chocolate with the butter, stirring until smooth. Remove from the heat and stir in the cocoa powder and the brown sugar. Mix in the eggs, one at a time.
Divide the chocolate mixture among the custard cups and bake for 15 minutes, or until the sides are firm but the center is still shiny and quite jiggly. Let the cakes cool for at least 5 minutes before serving.