Preheat oven to 350. Butter a 9 inch springform cake pan and line bottom with a circle of nonstick parchment paper.
To make cake, beat butter and sugar together in large bowl til light and creamy, then add the lemon zest.
Beat eggs in a small bowl. Mix flour and baking powder together in another bowl. Beat alternate spoonfuls of egg, then flour into the creamed mixture. Beat well after each addition til the mixture is smooth.
Peel, core and slice the apples. Toss with lemon juice, then stir two thirds into the cake mixture. Spoon into the prepared pan.
Spread top of cake level then sprinkle with remaining apple slices. Brush with melted butter and sprinkle with 1 Tbsp. sugar. Cook in the preheated oven for 55 to 65 minutes, til golden brown and a skewer inserted into center of cake comes out cleanly. You may need to cover the top of the cake with a piece of foil toward the end of cooking if it seems to be overbrowning.
Let cake cool for 10 minutes in the pan, then loosen the edge and remove the pan. Peel away the parchment paper and put onto a cooling rack. Serve warm or cold, cut into wedges, with a drizzle of toffee sauce
To make toffee sauce, heat 4 Tbsp. each of butter, corn syrup and sugar over a low heat, stirring til melted. Increase heat slightly and cook for 1 to 2 minutes, til the mixture begins to darken slightly in color. Remove from heat, gradually stir in 1 cup heavy cream, then return to stove. Cook for 2 to 3 minutes stirring all the time. Mix in 2 Tbsp. lemon juice and let cool.