Preheat the oven to 350°F. Line 2 baking sheets with parchment paper or silicone baking mats.
Sift the flour and baking soda into a small bowl and stir together.
In a small glass bowl or measuring cup, combine up to 2 tablespoons of the reserved bacon fat with enough melted butter for 8 tablespoons (½ cup) total. If the mixture cools, warm it in the microwave.
In a large bowl, whisk together the granulated sugar, brown sugar, salt, and warm butter and bacon fat mixture until smooth and there is no oily film left around the edges.
Whisk in the egg and vanilla, adding a little air, until the mixture is smooth and is light in colour.
Stir the flour mixture, chocolate chunks, and bacon chunks into the wet ingredients and mix until no dry flour remains. Be careful not to overmix.
Scoop thirteen 1½-tablespoon portions of cookie dough onto each prepared baking sheet, leaving ample space between them. Sprinkle the flaky sea salt (if using) over the cookies before baking. Bake, one sheet at a time, for 12 to 15 minutes, or until the cookies are lightly brown and golden brown around the edges. Allow the cookies to cool on the baking sheets. Store in an airtight container at room temperature for up to 5 days.