Preheat oven to 325C. Melt chocolate in double boiler – set aside to cool.
Cream butter and sugar in stand-up mixer. Add salt and eggs and beat well after each addition. Blend in vanilla and espresso powder and then flour (do not over-mix!). Stir in melted, cooled chocolate and chocolate chips or chunks.
Pour into parchment-lined, greased 8 x 8 x 2-inch pan and bake for around 30 – 35 minutes, until top becomes slightly lighter and cracked.