Preheat the oven to 350°F and line an 8x8-inch baking dish with parchment paper, leaving overhang on the sides for easy removal.
In a large bowl, stir together the flour, large flake oats, graham cracker crumbs, baking powder, baking soda, and salt. Set aside.
In a separate medium-sized bowl, beat the sugar and melted butter with a hand mixer on medium-high speed until it is smooth and combined. Add the eggs and vanilla extract and mix until fully incorporated.
Add the dry ingredients to the wet ingredients and stir with a wooden spoon until a thick dough forms. Press the cookie bar dough evenly into the prepared baking dish. If the dough is sticky, lightly dust your hands with flour to help press it flat and smooth.
Pour the sweetened condensed milk evenly over the cookie base and gently spread it into a thin layer using a spatula. Sprinkle the mini eggs and chocolate chips evenly over the top. Bake for 40-45 minutes, or until the top is golden brown and the center is set (the center should be soft, but not jiggly).
Allow the bars to cool completely in the pan before lifting them out. Transfer to a cutting board and slice into 16 squares.
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Notes
Graham Cracker Crumbs - These can be found in the baking aisle at most grocery stores. If you can't find them, pulse whole graham crackers in a food processor until they are a sand-like consistency.
Mini Eggs - If mini eggs are out of season or you can't find them, you can use M&M's, Smarties, Reese's Pieces or chocolate chunks.
Do not overbake the bars. They will finish cooking and firm up as they cool in the pan.
Use parchment paper and let it overhang the sides of the pan. The sweetened condensed milk turns very sticky during baking, and the bars will stick to the pan if you do not line it with parchment paper.
Use your hands or the bottom of a heavy-bottomed glass to press the cookie batter into an even, firm layer in the baking dish.
Store mini egg cookie bars in an airtight container at room temperature for up to 4 days or freeze for up to 2 months.