FIRST Preheat oven to 325 degrees and grease your Michigan shaped cast-iron skillet.
NEXT In a large bowl, beat butter and brown sugar with a mixer until fluffy, 3 to 4 minutes, stopping to scrape sides of the bowl. Add eggs and vanilla, beating to combine.
FOLLOWING In a medium bowl, whisk together flour, baking powder, and salt. With the mixer on low speed, gradually add flour mixture to butter mixture, beating just until combined. Stir in oats and toffee bits. Press dough into the bottom and corners of the prepared skillet and sprinkle with chopped toffee.
FINISH Bake until golden brown, 35-40 minutes, loosely covered with foil to prevent excess browning, if necessary.
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