Lightly coat a medium skillet with nonstick cooking spray and set over medium-high heat. When the skillet is hot, add the sausage and cook, stirring or turning over often, until browned and just cooked through, 4 to 6 minutes. Re-move from the heat and let cool.
Lightly coat a large skillet with nonstick cooking spray and set over medium to medium-high heat. Lay the bacon in a single layer in the skillet and cook, flipping once, until browned, about 3 minutes on each side. Be very careful not to burn the bacon or the bacon fat or it will turn your gravy dark. If you are nervous about it, take your time and cook the bacon low and slow for 4 to 5 minutes on each side. (I've messed this part up so many times by being in a hurry.) Once the bacon is cooked, transfer it to a plate, leaving the fat behind in the pan.
Turn the heat to medium-low; add the flour, bouillon powder, and ½ teaspoon pepper to the bacon fat; and whisk constantly until the flour just begins to brown and no longer tastes floury, about 2 minutes. Add the milk, ½ cup or so at a time, whisking constantly to incorporate after each addition. When all the milk is incorporated, continue to whisk until thickened to your desired consistency, about 4 minutes for a fairly thick gravy.
Crumble the cooled bacon and sausage with your hands, stir into the gravy, turn the heat to medium, and warm through, about 1 minute. Taste for seasoning and adjust with salt and black pepper, if needed. If your gravy is too thick, you can stir in a few tablespoons milk to thin it. If you want the gravy thicker, you can continue to simmer it over low heat to reduce.
When ready to serve, brush the tops of the biscuits with the butter, then split the biscuits open and pour the gravy over the tops, or, if you like, put the gravy in the middle, like a sandwich.
Leftover gravy will keep in an airtight container in the refrigerator for up to 3 days.
