I got this HTTP-API documentation by reversing the kptncook app and analysing its traffic.
Check out https://medium.com/analytics-vidhya/reversing-and-analyzing-the-cooking-app-kptncook-my-recipe-collection-5b5b04e5a085 for a guide on how to get started with the kpntcook app.

The API is accessible under https://mobile.kptncook.com

Each call to the api (aswell as calls to get objects such as recipe images from cloudfront) have to include a http param called kptnkey. This kptnkey is hardcoded in the app and doesn't seem to change.

I'm not sure what the implications of providing such an API key are so I won't provide it here. You can easily get it by analyzing the app in android-studio or just by searching kptnkey on github.

Then just append this key to each call like this: https://mobile.kptncook.com/<ressource_id>/?kptnkey=6*****y-o**k-I*****J-j****6

Logs user in and returns user info, the user's access token, name and the user's favorite recipe's ids.

• kptnkey required:
  No
• URL:
  /login/userpass
• Method:
  POST
• Request Body:
  type: json
  example:

  {
      "email": "<account_address>",
      "password": "<cleartext_password>"
  }

• Request Answer:
  example:

  {
      "accessToken": "1*****5-8**3-4**1-5**b-e*********d",
      "name": "<realname>",
      "favspace": 35,
      "favorites": [
          "5dcbd5cc5400009587b05515",
          ...
      ],
      "inviteCode": "4****4"
  }

Gets all favorites of the logged in user and returns their IDs as a list

• kptnkey required:
  No
• URL:
  /favorites
• Method:
  GET
• Request Headers:
  Token: insert the accessToken returned when logging in here
  example: Token: 1*****5-8**3-4**1-5**b-e*********d
• Request Answer:
  example:

  {
      "favspace": 35,
      "favorites": [
          "52c5e4021d46335f01572524",
          ...
      ]
  }

Returns one recipe by its id. You can search by a recipe's uid or identifier:

• Possible ids:
  • uid: Is the id which can be extracted from share links
    e.g. http://mobile.kptncook.com/recipe/pinterest/Low-Carb-Tarte-Flamb%C3%A9e-with-Serrano-Ham-%26-Cream-Cheese/315c3c32)
    The 315c3c32 part at the end is the uid
  • identifier: Is the internal id which is returned by the other api functions in the $oid field
• kptnkey required:
  Yes
• URL:
  /recipes/search
• Method:
  POST
• Request Parameters:
  • kptnkey
    example: ?kptnkey=6*****y-o**k-I*****J-j****6
  • lang
    possible values: en, de, fr, pt, es note: When using the lang parameter the api response switches to a different (new) format.
• Request Body:
  type: json
  example:

  [
      {
          "uid": "315c3c32"
      }
  ]

  or

  [
      {
          "identifier": "59b68c8d2c00000b0535125a"
      }
  ]

• Full Request Example:

  curl --location --request POST 'https://mobile.kptncook.com/recipes/search?kptnkey=6*****y-o**k-I*****J-j****6' \
  --header 'Token: 1*****5-8**3-4**1-5**b-e*********d' \
  --header 'Content-Type: application/json' \
  --data-raw '[
          {
              "uid": "315c3c32"
          }
          ]
  '

• Request Answer:
  example (old):

  [
      {
          "_id": {
              "$oid": "59b68c8d2c00000b0535125a"
          },
          "localizedTitle": {
              ...
              "en": "Low Carb Tarte Flambée with Serrano Ham & Cream Cheese",
              ...
          },
          ...
      },
      ...
  ]

  example (new with lang set):

  [
      {
          "_id": {
              "$oid": "59b68c8d2c00000b0535125a"
          },
          "title": "Low Carb Tarte Flambée with Serrano Ham & Cream Cheese",
          "rtype": "Pork",
          "gdocs": "1046",
          "authorComment": "Flourless but so delicious!",
          "uid": "315c3c32",
          ...
      },
      ...
  ]

Returns the 3 daily featured recipes in full detail

• kptnkey required:
  Yes
• URL:
  /reciped/de/<unix_timestamp>
  The timestamp doesn't actually matter, it always returns the 3 current recipes
  (a value of 1 can also be used)
• Method:
  GET
• Request Parameters:
  kptnkey
  example: ?kptnkey=6*****y-o**k-I*****J-j****6
• Request Answer:
  example:

  [
      {
          "_id": {
              "$oid": "60b0c1323c000035008250c9"
          },
          "localizedTitle": {
              ...
              "en": "Cheesy Chicken Bake with Garlic Croutons",
              ...
          },
          ...
      },
      ...
  ]