Butter and lightly flour 6 (6-ounce) ramekins. Tap out any excess flour.
In a small saucepan, bring to a boil 2 inches of water. In a large glass bowl, add butter, chocolate and espresso powder. Lower heat and place bowl over the simmering water. Slowly whisk mixture until the chocolate is almost completely melted. Remove bowl from heat and continue to whisk until all chocolate has melted. Allow to cool slightly.
In a large bowl, add eggs, yolks, and sugar. Using an electric mixer, beat until light and fluffy and pale in color. Reduce speed to medium-low and slowly pour in the chocolate mixture. Add the flour and mix until just combined. Do not overwork the batter.
Preheat oven to 450 degrees F.
Divide the batter between the molds. Line a baking sheet with parchment paper. Place chocolate molds onto baking sheet and place in oven for 7 minutes. If not baking immediately, cover chocolate molds with plastic wrap and refrigerate until ready to serve, up to 8 hours. When ready to bake, place refrigerated chocolate molds onto baking sheet and place in oven for 10 minutes. The top and sides should be cooked and dry and the inside will be runny.
Using oven mitts, carefully invert each mold onto a plate and let it sit for 10 seconds. Then lift 1 corner of the ramekin and the cake should slip out. Sprinkle about 1/2 teaspoon Frangelico over each cake. Spoon berries around the chocolate cake and garnish with mint. Serve warm.