Place chocolate chips and butter in a small bowl. Microwave for 10 seconds to melt. Blend together with a spoon. When there are unmelted chips, replace in the microwave for another 10 seconds. Rather than using a microwave, melt the chips and butter with a double boiler.
Spill the butter and chocolate chips mixture into a large mixing bowl. Crack the eggs into the bowl too. Mix along with the nutmeg, cocoa powder, sweetener, and vanilla. Blend to make a smooth, homogeneous mixture then add the peanut butter. Whisk again.
Pour the batter into a mold brushed with oil or butter. Turn the oven on to 350°F and bake for half an hour. Insert a toothpick into the brownies to check if cooked. If the toothpick comes out clean, remove the mold from the oven.
Mix the chocolate chips, sweetener, heavy cream (if using) and peanut butter for the drizzle. Melt together and stir until smooth and creamy. Drip the mixture onto the brownies. Chill the brownies in the fridge until the drizzle sets. Take out after 15 minutes and divide into square slices.