Preheat your oven to 350 F and line an 8" x 8" square cake pan with parchment paper.
In a large mixing bowl combine the flour, oats, graham cracker crumbs, baking powder, baking soda, and salt. Stir to combine and set aside.
In a medium bowl combine the sugar and melted butter, using a hand mixer or whisk beat until smooth. Add the eggs and vanilla and blend until smooth.
Pour the wet ingredients into the dry and stir with a wooden spoon or spatula until thick and well combined.
Press 1/2 of the cookie dough into the prepared cake pan and flatten so the surface is smooth and even. Add a bit of flour onto your hands if the dough is sticky.
Scatter the toffee pieces and press into the cookie dough. Top with the remaining dough, smoothing out to ensure it covers the toffee completely.
Top with Cadbury mini eggs then pour over the condensed milk.
Bake for 45 minutes.
Take the pan out of the oven then let the mini egg cookie bars cool completely before inverting onto a cooling rack.
Once the bars have cooled to room temperature, peel off the parchment paper. Cut the cookie bars into 16 pieces and serve.