Add lemon juice and dark brown sugar to boston shaker, stir until partial dissolved
Add the rums and stir again until sugar is completely dissolved
Fill shaker with ice and stir in beer, pour unstrained into a pint glass
NOTES: Maggie's Farm Rum uses Straub Amber Lager and their own brand of rums. I've also seen recipes using Red Stripe Lager, Privateer Amber rum, and White Virgin Islands rum.