Preheat the oven to 350 degrees F. Butter 4 (6-ounce) ramekins and set aside.
Over a double boiler, melt the butter and chocolate, whisking until smooth. Whisk in the sugar and transfer to a medium bowl. Whisk in the eggs and flour until smooth. Pour the batter into the prepared ramekins about 2/3 full. Place 1 of the small chocolate squares right in the middle of each cake and press down with your fingers so it rests in the middle of the ramekin. Bake the cakes until slightly puffy and set around the edges, 20 to 25 minutes. Don't worry if your cake tester comes out wet, the center should still be loose.
BYOC: Use a nice sea salt to garnish, or try toasted coconut flakes, toasted and chopped nuts, fresh berries in the summer or freshly whipped cream.