Jambon-Beurre (Classic Butter and Ham Baguette)

Leave 50 g unsalted butter at room temperature until a fingertip presses into it easily. Trying to force cold butter across the bread tears the crumb and leaves hard, uneven patches. Slice 4 cornichons thinly lengthwise and pat away their surface moisture. Set 1 baguette flat and split it lengthwise with the knife held low and steady. Open it gently by hand, preserving the interior rather than compressing or hollowing it, and brush away only large loose crumbs. Spread the softened butter over both cut faces in several thin passes. On one buttered side, apply 1 teaspoon Dijon mustard in a very thin layer from end to end, then scatter 1/4 teaspoon black pepper broadly. Fold 120 g jambon ham loosely onto the bread so it forms thin, airy layers instead of one dense stack. Tuck the cornichon slices between folds and near the edges, distributing their acidity through the full length of the sandwich. Close the baguette and press with your palm for only about 2 seconds, just enough to keep the filling in place without flattening the crumb. Cut it in half and serve immediately. If the assembled sandwich has been refrigerated, leave it at room temperature for 10 minutes before eating so the butter softens again rather than remaining a cold, firm layer against the ham.