Prep the crock. Line with a slow cooker liner or parchment, or grease well. If using a loaf pan, coil foil into a ring and set it on the bottom of the crock as a trivet.

Whisk the dry ingredients. Combine flour, baking soda, baking powder, salt, and all spices in a medium bowl. Whisk 15 seconds.

Whisk the wet ingredients. In a large bowl, whisk pumpkin, both sugars, oil, eggs, and vanilla until completely smooth.

Combine. Fold dry into wet with a spatula just until no dry flour remains. Do not overmix — small lumps are fine.

Transfer. Pour into the greased loaf pan and set it on the foil trivet, or pour directly into the lined crock and smooth the top.

Add streusel if using: rub the streusel ingredients together with your fingertips until crumbly and scatter over the batter.

Cover with a towel. Lay two paper towels or a clean kitchen towel across the crock, then set the lid on top. This catches condensation and keeps the top from going soggy.

Cook on HIGH. 2 to 2½ hours in a loaf pan, or 1¾ to 2¼ hours directly in the crock. Don't lift the lid for the first 90 minutes.

Test. Done at 200–205°F internal, or when a toothpick comes out with moist crumbs.

Rest. Turn off, uncover, and let sit 20 minutes before lifting out. Cool another 20 minutes before slicing.