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You should melt the butter and chocolate in a pot over the stove. Use a heavy sauce pan and melt the butter and chocolate over medium-high heat. Watch the pot closely to make sure nothing burns.  It's a good idea to remove the butter while it's still somewhat solid. Butter can burn easily. If the chocolate is melted and there is still a small chunk of butter that is solid, remove the pan from the heat. Mix everything together until the butter melts as well. After melting, allow your butter and chocolate mixture to cool off until it reaches room temperature. This should take about 15 minutes. Once the butter and chocolate mixture is cool, add your sugar. Use a mixing spoon to stir in the sugar. Keep stirring until you have an even mixture. Start with the eggs, adding them one at a time. Use a wire whisk to gently whisk each egg into the batter until the mixture is smooth and even. Once the eggs are added, whisk in the vanilla. To fold in the ingredients, add the flour and salt to the top of the butter mixture. From there, use a rubber spatula. Starting at the back of the bowl, cut a vertical line through the ingredients across the bottom of the bowl. Rotate the bowl a quarter of a circle and cut another vertical line using your spatula. Continue this pattern. Eventually, the ingredients should all mix together evenly.
Melt the butter and chocolate. Stir in the sugar. Whisk in the eggs and vanilla. Fold in the flour and salt.