Preheat the oven to 170C fan-forced. Lightly grease a 34 x 20cm slice pan and line with baking paper. Prepare the base: combine the flour, baking powder, brown sugar and salt in a mixing bowl, use your clean hands to rub the ingredients together and remove any lumps. Make a well in the centre, add the melted butter, egg, and vanilla and stir until just combined. Note: the mixture will look greasy at this stage but carry on as all will be good in the end. Transfer batter to the pan and spread into an even layer - I find a short offset palette knife makes this job easy. Place in the oven and bake for 12 minutes (note: the base will only be partially cooked at this stage), remove it from the oven and pour over the pecan topping.
Meanwhile, prepare the pecan topping. Combine the butter, brown sugar, maple syrup, cream and salt in a small saucepan and melt together over low to medium heat then set it aside to cool slightly. Add the eggs, vanilla and lemon juice and whisk to combine. Add the pecans and mix through.
Pour the topping mixture over the partially cooked base, return it to the oven and bake for a further 20 minutes or until golden and firm to the touch. When it’s ready, remove it from the oven and place it onto a cooling rack. When cool, lift the slice out of its pan and slice it into 18 bars.