Melt butter and chocolate in a large bowl in the microwave for 1 minute. Stir, continue heating at 20 second intervals until chocolate is completed melted and smooth. Allow to cool.
Preheat oven to 350 F and line an 8x8 baking pan with parchment paper, allowing parchment to hang over the sides.
Add sugar to the chocolate and stir until combined. Add eggs and beat with wooden spoon or rubber spatula until combined. Stir in vanilla. Stir in the flour and baking soda just until combined.
Spread the batter into the prepared pan and bake for 30 minutes. Allow to cool completely in the pan on a wire rack.