Preheat the oven to 350F/175C and line a 9" square baking tin with parchment paper. Stir the butter and crushed graham crackers together until the crumbs have absorbed all the liquid. Tip the crumbs into the lined baking tin and press down with the back of a spoon to form the base.
Place the baking tin in the preheated oven and bake for 15 minutes. Remove from the oven and set to one side.
While the base is baking, beat the cream cheese, yogurt, eggs, sugar, lemon zest and lemon juice together until totally combined. Pour onto the baked base and return to the oven for another 25 minutes, until the edges are fully set, but the centre still wobbles a little bit.
Remove the bars from the oven and leave to cool for at least 30 minutes at room temperature. Transfer the bars to the fridge to chill for a minimum of 4 hours then slice into 9 squares. Top with lemon slices before serving, if desired.