Step #1 Place lemon zest & the raisins in a small bowl & cover with 1/4 c of whiskey.
Step #2 Cover up with plastic wrap & leave to soak overnight.
Step #3 When ready to prepare the cake, lightly butter a 7-inch square cake pan.
Step #4 Line the base with parchment paper.
Step #5 Preheat the oven to 350°F.
Step #6 Place the butter in the bowl of an electric mixer (or mix by hand).
Step #7 Beat the butter until creamy.
Step #8 Add the sugar & continue beating until the mixture is light & creamy.
Step #9 Add the egg yolks & beat them into the mixture one at a time.
Step #10 Remove the lemon zest from the raisin & whiskey mixture.
Step #11 Chop some of the zest very finely, about 1 Tbsp, & add to the egg & sugar mixture along with the raisins & whiskey.
Step #12 Gently stir in theto the mixture.
Step #13 In another bowl, whisk the egg whites until stiff.
Step #14 Fold them into the egg & sugar mixture alternately with the flour.
Step #15 Don't overmix.
Step #16 Transfer the batter to the prepared pan & smooth the surface.
Step #17 Sprinkle top with the sugar & bake in the oven for about 20-25 mins or until it springs back carefully when touched & a tester emerges clean.
Step #18 Remove from the oven & let sit for about 10 mins.
Step #19 Turn out onto a cake rack set over a plate so that the cake is upside down.
Step #20 Use a skewer to make about 6 or 8 little holes in the cake's base (don't go all the way through) & pour the remaining bit of whiskey into the holes.
Step #21 Let sit for another 15 mins, then invert & let cool completely before cutting into thick fingers & serving.