Line a 5- by 9-in. loaf pan with plastic wrap, leaving at least a 1-in. overhang on all sides. Combine mascarpone, cream, and Kahlúa and beat on low speed, then medium, until mixture is thick enough to spread (do not overbeat).
Set 1 ladyfinger cookie vertically against a long edge of loaf pan and trim it flush with rim of pan. Use it as a guide to trim 12 more cookies, reserving trimmed ends. Soak 1 ladyfinger at a time in beer, about 1 second per side; then arrange in rows along the long sides of pan, picket fence–style. Arrange 4 ladyfingers lengthwise on bottom of pan.
Spoon half the mascarpone mixture over bottom layer of ladyfingers and smooth evenly. Top with another layer of soaked ladyfingers (using reserved trimmed ends first), then top with remaining mascarpone mixture, smoothing evenly. Sift cocoa over top of cream. Wrap beeramisu and chill at least 8 hours and up to
To serve, use plastic wrap to lift cake out of the loaf pan. Unwrap and slice crosswise. Dust each slice with additional cocoa, if you like.