Grease a 4 to 6-quart slow cooker with butter or spray with coconut oil.

In small saucepan, melt butter and unsweetened chocolate over low heat, whisking occasionally. Remove from heat and allow to cool.

Whisk together heavy cream, coffee crystals, and vanilla extract in a small bowl.

Combine cocoa, almond flour, and salt in a small bowl.

Beat eggs in a large bowl with an electric mixer on high speed until slightly thickened. Gradually add granulated sweetener.

Beat on high speed until thick and pale yellow, about 5 minutes.

Turn mixer speed to low and slowly mix in melted butter and unsweetened chocolate mixture.

Stir in cocoa, almond flour and salt mixture.

Using mixer at medium speed, slowly add in cream, coffee, and vanilla mixture.

Pour batter into slow cooker.

Place a paper towel over the opening of the slow cooker before placing lid on top. The paper towel absorbs condensation.

Cook on low for 2-3 hours if using a 6-quart cooker or 2.5 to 3.5 hours if using a 4-quart cooker.

At the end of cooking, center will have a soft soufflé consistency while edges will be more cake-like. The center of the cake will be slightly damp on top, but will resist slightly when pressed. The center of cake needs to reach at least 160º F.

Serve garnished with whipped cream or low-carb ice cream, if desired.