Line a 8"x8" pan with parchment paper and preheat the oven to 350 degrees F.

Fit a heat proof bowl over a small pot half filled with water, creating a double boiler, and melt the butter and chocolate chips together in the bowl over a gentle simmer. You could also melt the butter and chocolate together in the microwave.

Stir together the sugar, cocoa powder, salt, and espresso powder in a small bowl.

When the butter and chocolate have melted, remove the bowl from on top of the pot and stir in the sugar mixture with a wooden spoon.

When that is well mixed, stir in the lightly beaten eggs and vanilla.

Mix the teff flour in well and pour the batter into the pan. Spread the batter in the pan with the wooden spoon.

Bake at 350 degrees F for 40 minutes or until a cake tester comes out clean.