In a large bowl, toss apples with applejack or brandy and lemon juice. Let stand 1 hour at room temperature.
For the batter, whisk egg, sugar, and lemon zest in a small mixing bowl until it lightens in color and forms a ribbon when trailed from beaters. Beat in applejack or brandy and then buttermilk and butter.
Sift flour with baking soda and salt. Fold into buttermilk mixture until smooth. If batter is too thick, add 1 to 2 more teaspoons buttermilk.
Heat about 2 inches of oil in a deep fryer to about 375F.
Drain apple slices and pat dry. Dip slices, 1 or 2 at a time, into batter to coat completely. Let excess batter drip back into the bowl Lower apple slices, 1 or 2 at a time, into the hot oil and fly until bottom of each slice is deep golden brown and coating is puffed, 2 to 3 minutes. Carefully turn slices and fry until evenly golden brown, 2 to 3 minutes each.
Drain on paper towels and serve, dusted with sugar.