Beat the butter with the sugars until pale and fluffy - about 3 minutes. In a large bowl, sieve the flour, cocoa, salt, and baking powder and mix together. Add the egg and vanilla to the butter and sugar mixture and beat to combine, then add the flour mixture in thirds, alternating with the stout. Beat just until combined then place into the fridge for at least 1 hour or overnight.
To make the brigadeiro: boil the stout over a low heat for about 8 minutes, until reduced by half. Add the condensed milk and cocoa powder and mix well to combine then cook for 15 minutes, or until the mixture has thickened. Keep stirring constantly so it doesn’t stick to the bottom of the pan!
When thickened, pour the mixture onto a greased plate and place into the fridge to cool.
When the brigadeiro has cooled, shape into small balls, about the size of a walnut.
Preheat oven to 200C/180C fan. Line a baking tray with parchment paper.
Using an ice cream scoop, scoop out balls of the cookie dough. Flatten slightly and add a ball of the brigadeiro to the middle, then bring the edges in to encase the brigadeiro. Repeat until all of the dough is used up.
Bake the cookies for 10 - 15 minutes. Allow to cool on the baking tray for 2-3 minutes, then transfer to a wire rack to cool slightly more before serving.