Preheat oven to 350°F degrees. Combine dry ingredients (except brown sugar) in a large bowl. Set aside.

Cream softened butter using the paddle attachment on your mixer approximately 30-60 seconds on medium speed until light and fluffy. Scrape down the bowl, add brown sugar and beat on medium speed for 5 minutes, pausing to scrape down the bowl several times, until light and fluffy.

Add in eggs one at a time, adding the vanilla with the second egg. Beat for a minute after adding each one and scraping down the bowl after the egg has incorporated.

With the mixer on low speed, add the dry ingredients 1 cup at a time. Scrape down the bowl as needed. Stop the mixer as soon as no dry streaks of flour remain in the bowl.

Lightly grease a sheet pan with non-stick spray or a paper towel with a bit of cooking oil on it.

Use your hands to firmly press about ⅔ of the oatmeal mixture into a very thin even layer at the bottom of the pan, making sure to get into all four corners. Use a small offset spatula to prevent a lip from forming at the edges of the pan.

Make fudge filling. In a small, heavy-bottomed saucepan over medium-low heat, stir together sweetened condensed milk, chocolate chips, salt, and butter. Continue stirring until no lumps left and remain and the chocolate is shiny and smooth. Adjust burner as needed, stirring frequently to prevent chocolate from burning. Turn off the heat, stir in walnuts and vanilla.

Pour chocolate mixture across the surface of the oatmeal layer in the pan, using a spatula to gently push it into all the corners right to the edge of the sheet pan.

Use your hands to tear, crumble, and pinch the remaining oatmeal cookie dough, dropping it in chunks across the chocolate fudge. Try to get some variety in size between chunks.

Bake in 350°F oven for 25-30 minutes until lightly golden brown on top. The chocolate fudge will have bubbled up slightly, rising to the lip of the sheet pan.

Let the tray cool completely before cutting into diamonds. To get the diamond shape: cut 7 straight lines lengthwise down the sheet tray about 1½" apart, then cut across those lines in a diagonal, starting with a corner-to-corner cut to make it easier to replicate the correct angle as you make the rest of your cuts. The diagonal cuts should be about 1½-2″ apart.

The bars can be a bit tricky and stubborn to remove from the pan. I recommend starting with the odd-shaped edge pieces and then working your way toward the center.