Preheat the oven to 350°F. Line a 9 x 9 inch pan with parchment paper.
Bring the beer to a boil and allow it to reduce by half, about 5 minutes. Cool completely.
In a large enough microwavable bowl, melt the chocolate and butter until the butter has melted, about a minute.
Remove the bowl from the microwave and continue to stir until chocolate is completely melted.
Pour the melted chocolate and butter into a large bowl or stand mixer. Add in the cooled beer, sugar, eggs and vanilla and beat until the sugar has dissolved.
Mix in the flour and baking soda. The batter should be glossy with no bubbles.
Pour the batter into the prepared pan and top with the chopped hazelnuts.
Bake for 45 minutes until a toothpick inserted in the center comes out clean.